O’MELVENY & Myers and New York’s 88-lawyer O’Sullivan, a leading firm in the private equity arena, have signed a letter of intent to merge, paving the way for a combination that would substantially boost Los Angeles-based O’Melveny’s New York presence and strike yet another name from the shrinking roster of mid-sized New York firms.

The merger is subject to the approval of both firms’ partnerships. O’Melveny chairman Arthur B. Culvahouse Jr. said O’Melveny’s 216 partners will vote on the merger by the end of July, with a two-thirds majority required to approve the pact. Mr. Culvahouse said he expected the merger to be approved without difficulty. O’Sullivan’s 25 partners will also vote at the end of July and are expected to unanimously support the merger, said O’Sullivan chairman John J. Suydam.