Former Everton and Sunderland midfielder Oster, 34, made 28 appearances for the Bees last season, but could not prevent them from being relegated from the Football League.
Goalkeeper O'Brien lost his place to Graham Stack and missed the climax to the campaign through injury.
Defender N'Toko moved to Underhill on a short-term contract in February and made just two appearances.
The Bees have also confirmed that Stack, David Stephens, Luke Gambin, Harry Crawford and Mauro Vilhete will be staying with the club as they bid to bounce straight back next season.
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- Graham Stack