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Mostly shortened to MO, or method of operation. It should come as no shock that this is direct from Latin, but came to English in 1600s. Modus is much more ancient, from PIE, med, to take appropriate measures.
Seems to me MO was commonly use in film noire. I think it is going out of favor.

I think the common usage has become simply MO, with the audience so inured to this that they no longer realize it's an initialism for a Latin phrase.