Sam Tillen hit an 88th minute equaliser to stretch Brentford's unbeaten league run to 15 games and increase Leyton Orient's winless run to five matches.
Tillen scored his first Bees goal from 10 yards after Matt Heywood's header.
It was harsh on the O's, who dominated most of the game and had gone ahead when Michael Simpson scored after Joe Keith flicked on Matt Lockwood's pass.
Bees keeper Stuart Nelson denied Wayne Corden and Gary Alexander and blocked a 75-yard Justin Miller clearance.
Leyton Orient: Garner, Miller, Lockwood, Tann, Saah, Corden, Simpson, Chambers, Keith (Barnard 79), Alexander, Connor (Ibehre 84).
Subs Not Used: Morris, McMahon, Demetriou.
Goals: Simpson 15.
Brentford: Nelson, Osborne (Carder-Andrews 66), Tillen, Heywood, Griffiths, Brooker, O'Connor, Pinault, Rhodes (Charles 73), Kuffour, Willock (Moore 46).
Subs Not Used: Masters, Mousinho.
Goals: Tillen 88.
Att: 5,420.
Ref: U Rennie (S Yorkshire).