Office Memorandums Basics
"Formerly prepared by the Interpretative Division, OMs were internal formal, legal opinions addressing an issue of significance, arising in litigation, The IRS has agreed to release OMs." Inventory of IRS Guidance Documents — A Draft, Tax Analysts, Tax Analysts (This article was published in Tax Notes Today on July 17, 2000. Click on Publications→Tax Notes.)
Abbreviation, Office Memorandums
Online Sources for Office Memorandum
Perhaps because the used of O.M.s dropped off in the 1980s, they are not often found in full text in tax databases.