Green’s Dictionary of Slang

1. (UK Und.) the distinguishing working style of a criminal or gang.

1953		Mad mag. Jan.–Feb. 6: I [...] had them run the I.B.M. on Taxiderm’s M.O.
1959		A. Zugsmith Beat Generation 62: It isn’t the same M.O., Captain. The Aspirin Kid goes after married women.
1969		B. Jackson Thief’s Primer 77: They knew he was a safe man. They had his M.O., everything, and whenever he’d bust a safe, they’d know.
1970		G.F. Newman Sir, You Bastard 64: Very few criminals ever changed their MO’s.
1981		J. Ellroy Brown’s Requiem 207: The Big Man is everywhere. He knows my M.O.!!!!
1995		M. Dibdin Dark Spectre (1996) 45: Then there’s the MO [...] This guy sounds like a violent slob, a wife-beater. You’d expect him to use a shotgun.
2007		N. ‘Razor’ Smith Raiders 19: He had a pretty clever MO when it came to blagging.
2011		J.J. Connolly Viva La Madness 386: ‘You defending him?’ ‘Just saying. It’s not his MO’.
2013		D. Whish-Wilson Zero at the Bone [ebook] The R&I Bank done over in Fremantle that morning had used the same MO, the same Harley in the getaway.
2017		V. McDermid Insidious Intent (2018) 209: Was this chance, or was it part of the killer’s MO?
2020		D. Winslow ‘Crime 101’ in Broken 76: ‘You gonna bring up your Superman theory again?’ ‘Same MO’.

2. way of thinking.

1955		E. Hunter Blackboard Jungle 197: I dig your MO, West.
1999		K. Sampson Powder 267: He knew their MO. He knew what they thought they were getting out of the deal.

3. as sense 1 above in non-criminal contexts, a regular way of life.

1995		J. Stahl Permanent Midnight 24: This was our MO — procto-opium parties and pan-fried Nazi banter.
2012		(con. 1980s) I. Welsh Skagboys 142: What ye up tae? —Hingin oot oan the mean streets ay Leith, nae changes in this cat’s MO.
2023		(con. 1962) J. Ellroy Enchanters 74: ‘Peter will be all over the waitresses. It’s his well-established MO.’.

4. see mota n.