Green’s Dictionary of Slang

(US) a brief glance, a visual assessment.

1913		T.A. Dorgan in Zwilling TAD Lex. (1993) 61: Think I’ll go in and give the jury the once over. Think I’ll give them the o.o. myself.
1916		O.O. McIntyre New York Day By Day 5 Sept. [synd. col.] After giving the town the once over, or as Tad puts it, the ‘O.O.’ [etc].
1924		Hecht & Bodenheim Cutie 20: ‘Say, bozo,’ Cutie said, handing him an O.O. [...] what kind of racket is this?’.
1950		Goldin et al. DAUL 149/1: O. O., the. The once-over; a quick searching glance.