Document:

<PAGE>   1
                                                                   EXHIBIT 10.15
                          OPTIMARK TECHNOLOGIES, INC.

                            STOCK PURCHASE AGREEMENT

         THIS AGREEMENT is made as of  December 18, 1998, between OptiMark
Technologies, Inc., a Delaware corporation (the "Company"), and Phillip J.
Riese (the "Purchaser").

         WHEREAS in order to give the Purchaser an opportunity to acquire an
equity interest in the Company as an incentive for the Purchaser to participate
in the affairs of the Company, the Company is willing to sell to the Purchaser
and the Purchaser desires to purchase shares of Common Stock according to the
terms and conditions contained in the Employment Agreement between the
Purchaser and the Company effective as of November 1, 1998 (the "Employment
Agreement") and the Amended and Restated Stockholders Agreement dated April 23,
1998, as amended, among the Company and the signatories to such Agreement (the
"Stockholders Agreement") and herein.

         THEREFORE, the parties agree as follows:

         1.      Sale of Stock.  The Company hereby agrees to sell to the
Purchaser and the Purchaser hereby agrees to purchase an aggregate of 250,000
shares of the Company's Common Stock, par value $.01 per share (the "Shares"),
at the price of $10.00 per share for an aggregate purchase price of $2,500,000.

         2.      Payment of Purchase Price.  The purchase price for the Shares
may be paid by delivery to the Company at the time of execution of this
Agreement of cash, check, or any combination thereof.

         3.      Investment Representations.  In connection with the purchase
of the Shares, the Purchaser represents to the Company the following:

                 (a)      The Purchaser is aware of the Company's business
affairs and financial condition and has acquired sufficient information about
the Company to reach an informed and knowledgeable decision to acquire the
securities.  The Purchaser is purchasing these securities for investment for
the Purchaser's own account only and not with a view to, or for resale in
connection with, any "distribution" thereof within the meaning of the
Securities Act of 1933, as amended (the "Securities Act").

                 (b)      Purchaser acknowledges and understands that the
Securities constitute "restricted securities" under the Securities Act and have
not been registered under the Securities Act in reliance upon a specific
exemption therefrom, which exemption depends upon, among other things, the bona
fide nature of Purchaser's investment intent as expressed herein.  Purchaser
further understands that the Securities must be held indefinitely unless they
are subsequently registered under the Securities Act or an exemption from such
registration is available.  Purchaser further acknowledges and understands that
the Company is under no obligation to register the Securities.

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         4.      Stock Certificate Legends.  Purchaser understands and agrees
that the Company shall cause the legend set forth below or a legend
substantially equivalent thereto, to be placed upon any certificate(s)
evidencing ownership of the Shares together with any other legends that may be
required by the Company or by applicable state or federal securities laws:

                 THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED
                 UNDER THE SECURITIES ACT OF 1933 (THE "ACT") OR THE SECURITIES
                 LAWS OF ANY STATE.  THE SECURITIES MAY NOT BE TRANSFERRED
                 EXCEPT PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT UNDER
                 THE ACT AND APPLICABLE SECURITIES LAWS OR PURSUANT TO A
                 WRITTEN OPINION OF COUNSEL REASONABLY SATISFACTORY TO THE
                 COMPANY THAT SUCH REGISTRATION IS NOT REQUIRED.

         5.      Tax Consequences.  The Purchaser has reviewed with the
Purchaser's own tax advisors the federal, state, local and foreign tax
consequences of this investment and the transactions contemplated by this
Agreement. The Purchaser is relying solely on such advisors and not on any
statements or representations of the Company or any of its agents.  The
Purchaser understands that the Purchaser (and not the Company) shall be
responsible for the Purchaser's own tax liability that may arise as a result of
this investment or the transactions contemplated by this Agreement.

         6.      General Provisions.

                 (a)      This Agreement shall be governed by the laws of the
State of New York as they apply to contracts entered into and wholly to be
performed in such state.  This Agreement, together with the Employment
Agreement and the Stockholders Agreement,  represent the entire agreement
between the parties with respect to the purchase of Common Stock by the
Purchaser and may only be modified or amended in writing signed by both
parties.

                 (b)      All notices, demands or other communications provided
for or permitted hereunder shall be made in writing and shall be registered or
certified first class mail, return receipt requested, courier service, outright
mail or personal delivery:

                          If to the Company:
                          ------------------

                          OptiMark Technologies, Inc.
                          530 Main Avenue
                          Durango, CO  81301
                          Attention:  William A. Lupien

                          If to the Purchaser:
                          ---------------------

                          Phillip J. Riese

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                          141 Prince Street, Apt. 4
                          New York, NY  10012

                 (c)      The rights and benefits of the Company under this
Agreement shall be transferable in the case of a sale of substantially all of
the assets of the Company to any one or more persons or entities, and all
covenants and agreements hereunder shall inure to the benefit of, and be
enforceable by the Company's successors and assigns.  The rights and
obligations of the Purchaser under this Agreement may only be assigned with the
prior written consent of  the Company.

                 (d)      Either party's failure to enforce any provision or
provisions of this Agreement shall not in any way be construed as a waiver of
any such provision or provisions, nor prevent that party thereafter from
enforcing each and every other provision of this Agreement.  The rights granted
both parties herein are cumulative and shall not constitute a waiver of either
party's right to assert all other legal remedies available to it under the
circumstances.

                 (e)      The Purchaser agrees upon request to execute any
further documents or instruments necessary or desirable to carry out the
purposes or intent of this Agreement.

                 (f)      Purchaser acknowledges and agrees that this Agreement
and the transactions contemplated hereunder do not constitute an express or
implied promise of continued engagement as an employee or consultant for any
period, or at all, and shall not interfere with Purchaser's right or the
Company's right to terminate Purchaser's employment or consulting relationship
at any time, with or without cause.

                 (g)      Purchaser has reviewed this Agreement in its
entirety, has had an opportunity to obtain the advice of counsel prior to
executing this Agreement and fully understands all provisions of this
Agreement.

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                 (h)      This Agreement may be executed by either of the
parties hereto in one or more counterparts, none of which need contain the
signature of more than one party hereto, and each of which shall be deemed an
original, and all of which together shall constitute a single agreement.

         IN WITNESS WHEREOF, the parties have duly executed this Agreement as
of the day and year first set forth above.

<TABLE>
<S>                                                   <C>
OPTIMARK TECHNOLOGIES, INC.                                 PURCHASER:  Phillip J. Riese
a Delaware corporation

By:  /s/  William A. Lupien                                 By:  /s/  Phillip J. Riese
   ------------------------------------------------           -----------------------------------------------------
                                                            (Signature)

Title: Chairman
       --------------------------------------------

         (Type or Print Name)                               141 Prince Street
                                                            -------------------------------------------------------

                                                            New York, NY  10012
                                                            -------------------------------------------------------
                                                            (Address)
</TABLE><PAGE>   1
                                                                   EXHIBIT 10.16

                             STOCK OPTION AGREEMENT

         THIS AGREEMENT is made this 1st day of November, 1998, by and between
OptiMark Technologies, Inc., a Delaware corporation (hereinafter referred to as
"Employer"), and Phillip J. Riese (hereinafter referred to as "Employee").

         WHEREAS, Employee is a valued and trusted employee of Employer, or of
a subsidiary of Employer, and Employer considers it desirable and in its best
interest that Employee be given an inducement to acquire a proprietary interest
in Employer, as an added incentive to advance the interests of Employer, by
giving Employee an option to purchase common stock of Employer in accordance
with the Incentive Stock Option Plan (hereinafter referred to as the "Plan")
adopted by the Board of Directors of Employer's predecessor on July 15, 1994,
and approved by the shareholders of Employer's predecessor as of July 15, 1994,
and adopted by Employer as its own plan on July 29, 1996;

         NOW, THEREFORE, IT IS AGREED BY AND BETWEEN EMPLOYER AND EMPLOYEE AS
FOLLOWS:

     1.      OPTION GRANTED.  Employer hereby and herein grants Employee an
option to purchase 1,000,000 shares of OptiMark Technologies, Inc., common
stock, which shall become exercisable as to 200,000 of the option shares on or
after each of the five succeeding anniversary dates of the execution of this
Agreement, provided that Employee is employed by Employer or a subsidiary of
Employer (hereinafter also referred to as "Employer") on the applicable
anniversary date, beginning on November 1, 1999, and ending on November 1,
2003, in the manner described below and subject to the conditions contained
herein.  Subject to Section 6 herein, the option shall have a term of ten (10)
years, and notwithstanding anything in this Agreement to the contrary, no
option granted herein shall be exercisable at any time after ten (10) years
have passed from the date this option is granted.

     2.      EXERCISE PRICE OF OPTION.  Employee shall be entitled to exercise
the option granted herein at a purchase price of ten dollars ($10.00) per
share, said price being the fair market value of a share of Employer's common
stock on the date the option is granted.

     3.      CUMULATIVENESS OF OPTION.  The right to exercise the option
granted herein is cumulative, so that if Employee does not exercise his/her
option at the moment the option is first exercisable, as described in Paragraph
1 hereof, his/her right to exercise the same shall not lapse, but shall
continue, subject to the other conditions contained in this Agreement, until
such time as the option shall terminate, as described herein.

     4.      EXERCISE OF OPTION.  Subject to the other conditions contained in
this Agreement, exercise of the option granted herein shall be made by the
giving of written notice to Employer by Employee.  Such written notice shall be
deemed sufficient for purposes of this Agreement only if such notice is
delivered by registered or certified mail to Employer at its principal office,
states the number of shares with respect to which the option is being
exercised, and further

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states the date, not more than ninety (90) days after the date of such notice,
on which the shares of stock shall be taken up and payment therefor shall be
made.  If payment is not received within the ninety (90) days after the date of
such notice, the written notice shall be deemed null and void.

                 The payment for shares of stock taken up pursuant to an
exercise of the option granted herein shall be made in cash or certified check
at the principal office of Employer or at any office of a transfer agent
appointed for the shares of the stock of Employer.  Upon an exercise of the
option granted herein in compliance with the provisions of this paragraph, and
upon the receipt by Employer or its transfer agent of payment for the stock so
taken up, Employer shall deliver or cause to be delivered to Employee so
exercising his/her option a certificate or certificates for the number of
shares of stock with respect to which the option is so exercised and payment is
so made.

         Employee hereby agrees that, if so requested by Employer or any
representative of the underwriters (the "Managing Underwriter") in connection
with any registration of the offering of any securities of Employer under the
Securities Act of 1933, as amended (the "Securities Act"), Employee shall not
sell or otherwise transfer any option shares or any securities of Employer
during the 180-day period (or such other period as may be requested in writing
by the Managing Underwriter and agreed to in writing by Employer) (the "Market
Standoff Period") following the effective date of a registration statement of
Employer filed under the Securities Act.  Such restriction shall apply only on
the first registration statement of Employer to become effective under the
Securities Act that includes securities to be sold on behalf of Employer to the
public in an underwritten public offering under the Securities Act.  Employer
may impose stop-transfer instructions with respect to securities subject to the
foregoing restrictions until the end of such Market Standoff Period.

     5.      TRANSFER OF OPTION.  The option granted herein shall not be
transferred by Employee, other than by will or the laws of descent and
distribution, and shall be exercisable, during his/her lifetime, only by
Employee. Notwithstanding the above, in the event of Employee's death, the
representative of Employee's estate, or the person who received by bequest or
inheritance the right to exercise such option, may exercise the option to the
same extent as if the option were being exercised by the decedent, and subject
to the same conditions as the decedent, except as otherwise noted herein.

     6.      TERMINATION OF EMPLOYMENT.

          (a)        Upon termination of Employee's employment by the Employer
for other than Cause (as such term is defined in the employment agreement
between the Employer and the Employee effective as of November 1, 1998 (the
"Employment Agreement")) or by the Employee for Good Reason (as defined in the
Employment Agreement), (i) 60% of the option shall vest if such termination
occurs prior to November 1, 2001, (ii) 80% shall vest if such termination
occurs on or after November 1, 2001 but prior to November 1, 2002, and (iii)
100% shall vest if such termination occurs on or after November 1, 2002 and, in
each case, shall remain exercisable for 90 days following such termination.
For purposes of the preceding sentence, the applicable vesting percentage shall
include any portion of the option vested prior to such termination.

                                      -2-

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          (b)        Upon the termination of Employee's employment by the
Employer for Cause, disability or death, or by Employee without Good Reason,
any option, or part thereof, that is not exercisable as of the date the
Employee's employment so terminates (hereinafter referred to as "Termination
Date"), shall also terminate.  As to any option, or part thereof, which is
exercisable on the Termination Date:

                  (i)             in the event of Employee's disability (within
the meaning of Section 22(e)(3) of the Internal Revenue Code) while employed by
Employer, any option, or part thereof, granted to Employee under the Plan,
which is exercisable on the Termination Date and not previously exercised or
otherwise expired, shall be exercisable at any time within one (1) year from
the date Employee's employment so terminates;

                  (ii)            in the event of Employee's death while
employed by Employer, any option, or part thereof, granted to Employee under
the Plan, which is exercisable on the Termination Date and not previously
exercised or otherwise expired, shall be exercisable at any time within six (6)
calendar months from the date of Employee's death;

                  (iii)           in the event Employee's employment with
Employer is terminated by Employee without Good Reason, any option, or part
thereof, granted to Employee under the Plan, which is exercisable on the
Termination Date and not previously exercised or otherwise expired, shall be
exercisable at any time within ninety (90) days from the date employment so
terminates; or

                  (iv)            in the event Employee's employment with
Employer is terminated by Employer for Cause, any option, or part thereof,
granted to Employee under the Plan, which is exercisable on the Termination
Date and not previously exercised or otherwise expired, shall be exercisable at
any time within thirty (30) days from the date employment so terminates.

     7.      CHANGE OF CONTROL.

          (a)        In the event of a Change of Control (as defined in the
Employment Agreement), (i) 100% of the option shall vest if the Change of
Control occurs prior to November 1, 1999, (ii) 75% of the option shall vest if
the Change of Control occurs on or after November 1, 1999 but prior to November
1, 2000, (iii) 60% of the option shall vest if the Change of Control occurs on
or after November 1, 2000 but prior to November 1, 2001, (iv) 80% of the option
shall vest if the Change of Control occurs on or after November 1, 2001 but
prior to November 1, 2002, and (v) 100% of the option shall vest if the Change
of Control occurs on or after November 1, 2002.  For purposes of the preceding
sentence, the applicable vesting percentage shall include any portion of the
option that is vested prior to such Change of Control.

          (b)        In the event Employee's employment is terminated by
OptiMark (or its successor) without Cause or by Employee with Good Reason
within one year following a Change in Control, the option shall become fully
vested and exercisable, and shall remain exercisable for 90 days following such
termination.

     8.      OWNERSHIP OF STOCK.  Employee will not be deemed to be a holder of
any shares as to which this option is granted, and shall have none of the
rights of a shareholder as to any

                                      -3-

<PAGE>   4
of the shares as to which this option is granted, until payment of the option
price by him/her and delivery of a stock certificate to him/her for such
shares, and then shall be deemed a holder of shares with the corresponding
shareholder rights only as to the number of shares for which Employee has paid
and a stock certificate delivered.  Except as otherwise provided in this
Agreement, no adjustment shall be made for dividends or other rights for which
the record date is prior to the date such stock certificate is delivered.

                 All shares taken up by Employee pursuant to any exercise of
the option granted herein shall be registered in the name of Employee or in the
name of Employee jointly with his/her spouse.  Nothing contained herein,
however, shall be construed to prohibit any shares taken up by Employee
pursuant to any exercise of the option granted herein from being registered in
the name of a trust pursuant to a qualified 401(k) plan or qualified family,
living or similar trust wherein the beneficiary of such plan or trust is
Employee or Employee and his/her heirs.

     9.      LIMITATION ON EXERCISE.  The option granted herein may not be
exercised if the issuance of shares of common stock of Employer upon such
exercise would constitute a violation of any applicable Federal or State
securities or other laws.  Employee, as a condition to his/her exercise of this
option, shall represent to Employer that the shares of common stock of Employer
that Employee acquires under this option are being acquired by and for Employee
for investment and not with a present view to distribution or resale, unless
counsel for Employer is of the opinion that such a representation is not
required under the Securities Act of 1933 or any other applicable law,
regulation or rule of any governmental agency or private regulating body.
Furthermore, the option granted herein shall be subject to the requirement that
if at any time Employer shall determine, in its sole discretion, that the
listing, registration or qualification of the shares covered thereby under any
State or Federal law, or the consent of or approval of any governmental
regulatory body, is necessary or desirable as a condition of, or in connection
with, the granting of such option, or the issue or purchase thereunder, such
option shall not be exercised in whole or in part unless and until such
listing, registration, qualification, consent, or approval shall have been
effected or obtained free of any condition not acceptable to Employer.

     10.     EFFECTIVE DATE OF PLAN.  The Plan became effective in accordance
with Section 15 therein.

     11.     ACKNOWLEDGEMENT.  Employee acknowledges receipt of a copy of the
Plan, a copy of which is annexed hereto, and represents that he/she is familiar
with the terms and provisions thereof. The Plan is incorporated herein by
reference.  Employee hereby accepts this option subject to all the terms and
provisions of the Plan. Employee hereby agrees to accept, as binding,
conclusive and final, all decisions and interpretations of the Board of
Directors of Employer upon any question arising under the Plan.  The Plan, this
agreement and the Employment Agreement constitute the entire agreement of the
parties with respect to the subject matter hereof and supersede in their
entirety all prior undertakings and agreements of the Employer and Employee
with respect to the subject matter hereof, and may not be modified adversely to
the Employee's interest except by means of a writing signed by the Employer and
Employee.  As a condition to the issuance of shares of common stock of Employer
under this option, Employee authorizes Employer to withhold, in accordance with
applicable law, from any regular cash compensation payable to him, any taxes

                                      -4-

<PAGE>   5
required to be withheld by Employer under Federal, State or local law as a
result of his exercise of this option.  This agreement is governed by the
internal substantive laws, but not the choice of law rules, of Colorado.

                 Signed and sealed on the date first above written with intent
to be legally bound.

                                                EMPLOYER

                                                OPTIMARK TECHNOLOGIES, INC.

                                                By:  /s/  William A. Lupien
                                                Chairman

                                                EMPLOYEE

                                                By:  /s/  Phillip J. Riese

                                      -5-

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