Green’s Dictionary of Slang

m.o. n.

[abbr. Lat. modus operandi, the way of working]

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

[US]Mad mag. Jan.–Feb. 6: I [...] had them run the I.B.M. on Taxiderm’s M.O.
[US]A. Zugsmith Beat Generation 62: It isn’t the same M.O., Captain. The Aspirin Kid goes after married women.
[US]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.
[UK]G.F. Newman Sir, You Bastard 64: Very few criminals ever changed their MO’s.
[US]J. Ellroy Brown’s Requiem 207: The Big Man is everywhere. He knows my M.O.!!!!
[UK]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.
[UK]N. ‘Razor’ Smith Raiders 19: He had a pretty clever MO when it came to blagging.
[UK]J.J. Connolly Viva La Madness 386: ‘You defending him?’ ‘Just saying. It’s not his MO’.
[Aus]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.
[Scot]V. McDermid Insidious Intent (2018) 209: Was this chance, or was it part of the killer’s MO?
[US]D. Winslow ‘Crime 101’ in Broken 76: ‘You gonna bring up your Superman theory again?’ ‘Same MO’.
[Aus]D. Whish-Wilson I Am Already Dead 9: Their MO was about escalation – doing what most people wouldn’t do, so that a mere threat became effective.

2. way of thinking.

[US]E. Hunter Blackboard Jungle 197: I dig your MO, West.
[UK]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.

[US]J. Stahl Permanent Midnight 24: This was our MO — procto-opium parties and pan-fried Nazi banter.
[Scot](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.
[US](con. 1962) J. Ellroy Enchanters 74: ‘Peter will be all over the waitresses. It’s his well-established MO.’.

4. see mota n.