North End were outplayed in all departments and also ended the game with 10 men after a late red card for Shane Cansdell-Sherriff.
Westley said: "We have to take a long, hard look at ourselves because that performance was unacceptable.
"Some of our defending was shambolic and we all, as players and a coaching staff, need to look at why that was.
"It's a very bad day at the office and I obviously picked the wrong team because we didn't have the right energy levels.
"Oldham were the better side from start to finish. They deserved to take the lead when they did and we made a lot of defensive errors."