Striker Lewis Alessandra struck for his first goal of the season to put Oldham in front with a well placed shot following a quick break by Chris Taylor after 30 minutes.
Brighton forward Ashley Barnes spurned three chances and also struck the post before levelling a minute from the break by powerfully heading in a corner from Elliott Bennett.
Sandaza, recently released by Dundee United, then settled the issue in the fifth minute of stoppage time after a Bennett corner was headed back in by Inigo Calderon.
Barnes should have hit the target after only 10 minutes when put through by Gary Dicker, but keeper Dean Brill produced a stunning save diving to his right. Brill then denied the Seagulls again three minutes later when he thwarted Barnes after winger Kazenga Lua Lua's excellent cross.
Oldham made a bright start to the second half and Alessandra had a shot saved by Brighton keeper Casper Ankergren before Barnes' effort was deflected over the bar at the other end.