Green’s Dictionary of Slang

(1980s+) acro. that emphasizes the need to be seen in public gay environments, spec. Castro Street, the heart of gay San Francisco.

1985		‘Jennifer Blowdryer’ Modern English 7: BSOC (acronym) Once a week, if you’re a San Franciosco gay, you must BSOC (Be Seen On Castro).
1997		Fessler & Rauch When Drag Is Not a Car Race n.p.: BSOC Be seen on Castro; dated San Francisco term.