**Exhibit 31.2**

**CERTIFICATION**

I, Kathleen P. McSherry, certify that:

|   | 1.     | I have reviewed this   annual report on Form 10-K of the Ridgewood Energy O Fund, LLC;                                                                                                                                                                                                                                    |
|--------|--------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
|   |   |                                                                                                                                                                                                                                                                                                                      |
|   | 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(s) 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)) 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)     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 |
|   |   |                                                                                                                                                                                                                                                                                                                      |   |
|   |   |                                                                                                                                                                                                                                                                                                                      | (c)     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(s) 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 functions): |
|   |   |                                                                                                                                                                                                                                                                                                                      |
|   |   |                                                                                                                                                                                                                                                                                                                      | (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. |