**Exhibit 31.1**

**CERTIFICATION**

|   | I, James E. Declusin, certify   that:                                                                                                                                                                                                                                                                                                             |
|--------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
|   |                                                                                                                                                                                                                                                                                                                                              |
| 1.     | I have reviewed this quarterly   report on Form 10-Q of Oregon Steel Mills, Inc.;                                                                                                                                                                                                                                                                 |
|   |                                                                                                                                                                                                                                                                                                                                              |
| 2.     | Based on my knowledge, this   report does not contain any untrue statement of a material fact or omit to   state a material fact necessary to make the statements made, in light of the   circumstances under which such statements were made, not misleading with   respect to the period covered by this report;                                |
|   |                                                                                                                                                                                                                                                                                                                                              |
| 3.     | Based on my knowledge, the   financial statements, and other financial information included in this   report, fairly present in all material respects the financial condition,   results of operations and cash flows of the registrant as of, and for, the   periods presented in this report;                                                   |
|   |                                                                                                                                                                                                                                                                                                                                              |
| 4.     | The registrant’s other   certifying officer and I are responsible for establishing and maintaining   disclosure controls and procedures (as defined in Exchange Act Rules   13a-15(e) and 15d-15(e)) and internal control over financial reporting (as   defined in Exchange Act Rules 13a-15(f) and 15d-15(f)) for the registrant and have: |
|   |                                                                                                                                                                                                                                                                                                                                              |
|   | a)                                                                                                                                                                                                                                                                                                                                                | Designed such disclosure   controls and procedures, or caused such disclosure controls and procedures to   be designed under our supervision, to ensure that material information   relating to the registrant, including its consolidated subsidiaries, is made   known to us by others within those entities, particularly during the period   in which this report is being prepared; |
|   |                                                                                                                                                                                                                                                                                                                                              |   |
|   | b)                                                                                                                                                                                                                                                                                                                                                | Designed such internal control   over financial reporting, or caused such internal control over financial   reporting to be designed under our supervision, to provide reasonable   assurance regarding the reliability of financial reporting and the   preparation of financial statements for external purposes in accordance with   generally accepted accounting principles; |
|   |                                                                                                                                                                                                                                                                                                                                              |   |
|   | c)                                                                                                                                                                                                                                                                                                                                                | Evaluated the effectiveness of   the registrant’s disclosure controls and procedures and presented in this   report our conclusions about the effectiveness of the disclosure controls and   procedures, as of the end of the period covered by this report based on such   evaluation; and |
|   |                                                                                                                                                                                                                                                                                                                                              |   |
|   | d)                                                                                                                                                                                                                                                                                                                                                | Disclosed in this report any   change in the registrant’s internal control over financial reporting that   occurred during the registrant’s most recent fiscal quarter (the registrant’s   fourth fiscal quarter in the case of an annual report) that has materially   affected, or is reasonably likely to materially affect, the registrant’s   internal control over financial reporting; and |
|   |                                                                                                                                                                                                                                                                                                                                              |   |
| 5.     | The registrant’s other   certifying officer and I have disclosed, based on our most recent evaluation   of internal control over financial reporting, to the registrant’s auditors   and the audit committee of the registrant’s board of directors (or persons   performing the equivalent function):                             |
|   |                                                                                                                                                                                                                                                                                                                                              |
|   | a)                                                                                                                                                                                                                                                                                                                                                | All significant deficiencies   and material weaknesses in the design or operation of internal control over   financial reporting which are reasonably likely to adversely affect the   registrant’s ability to record, process, summarize and report financial   information; and |
|   |                                                                                                                                                                                                                                                                                                                                              |   |
|   | b)                                                                                                                                                                                                                                                                                                                                                | Any fraud, whether or not   material, that involves management or other employees who have a significant   role in the registrant’s internal control over financial reporting. |