Green’s Dictionary of Slang

(US) other people’s things, usu. money or alcohol; occas. as adj. (cf. o.p.’s n.).

1886		McAfee Kentucky Politicians 224: ‘O.P.’ – which means other people’s whisky – a brand which the most fastidious drinker rarely fails to take.
1902		‘Curlew’ ‘Boko’ in Bulletin Reciter 1880–1901 157: I started drinking at the shanty on the Flat / Where the o.p. grog is snaky.
1956		‘Toney Betts’ Across the Board 294: Touts work with other people’s money. O.P. That’s what they call it at the track.
1966		‘Minnesota Fats’ Bank Shot xiii: O.P. Money – Other People’s Money.