Ibrahima Konate’s nightmare display, featuring an own goal and poor defending for the Bees’ third goal, underlined a bad night for the Reds.
Oxlade-Chamberlain got his first Premier League goal in almost a year in reply but Liverpool were left four points off the top four as a result of the defeat.
“We just weren't good enough,” he told the club website. “We knew it was going to be a difficult game, we knew their threat around the set-pieces and we didn't do well enough.
“We got away with a couple with VAR ruling them out, but they were the warning signs and in the end we conceded a sloppy second goal after we sort of got away with that.
“Then at 2-0 it's difficult. I think at times we did well and we created some good moments but we didn't carry on doing the same things that got us that success. We came looking for the ball to feet a little bit too much I think and stopped offering in behind and causing them problems.
“At half-time we wanted to come out and really go for the game and believe. The early goal helped but unfortunately we couldn't get the second one in quick succession. If we had it might have been a different ending.
“Brentford play their game and they play it really well, and we kind of fell into the trap a little bit. Overall, it wasn't good enough from us.”