The Bantams, who face Aston Villa in the semi-final first leg of the Capital One Cup on Tuesday, looked off the pace and Atieno took full advantage by tapping in Elliot Johnson's cross four minutes before half time.
Oster then converted Ricky Holmes' low centre three minutes after the restart to earn the Bees a first win in three games.
Bradford's James Hanson almost scored an early goal but his third-minute shot was narrowly wide of Graham Stack's goal.
Gary Jones then got away an unchallenged header deep in the Barnet box from Stephen Darby's 38th-minute delivery, but he produced a simple save from Stack.
Despite their early dominance, Bradford's defence failed to cope with Johnson's low delivery which allowed Atieno to fire into the middle of the goal from two yards.
And Oster's effort secured all three points for Barnet in an entertaining second half.
The former Sunderland midfielder finished a clinical counter-attack following a horrible miss from Bradford's Nahki Wells.