o.p.m., n. — Green’s Dictionary of Slang

o.p.m. n.

[orig. US Und. and used by confidence tricksters of various types, it was a staple of City or Wall Street jargon by the 1980s]

other people’s money, the ideal commodity for a risky investment; also attrib.

1901		J. Flynt World of Graft 169: It’s cost me nothing to play the game, because I played it with O.P.M. (other people’s money).
[1904		Sydney Sportsman (Surry Hills, NSW) 5 Oct. 2/3: ‘Other People’s Money’ [...] is a musical farce about a millionaire, who gives his secretary advice to elope with a rich man's daughter, so the secretary takes steps to elope with her].
1935		A.J. Pollock Und. Speaks.
1969		Time 15 Aug. 60: No institution manages more ‘O.P.M.’, or Other People’s Money, than Manhattan’s 116-year-old United States Trust Co.
2006		M. Karp Rabbit Factory (2007) 146: Isn’t one supposed to be licensed to use OPM when you buy and sell stocks?
2007		Philadelphia Inquirer (PA) 8 Feb. A17/5: The o.p.m. (other people’s money) thing is upsetting. And shocking.