Steve MacLean's penalty rescued a 1-1 draw at home to Steve Coppell's men after Dave Kitson put the runaway leaders ahead in the first half.
And with The Owls safe from relegation, the home team played free and without caution - and were unlucky to not beat The Royals.
O'Brien told Footymad: "I thought in the second half we were different class. I know that the pressure is off us and all but we could have ended up winning it. I don't know why we played so free because we played well last week when the pressure was on us and managed to get two great results but I think over the last two or three months, we have put in lots of good performances. There's been the odd bad one but overall we have done well in the last few months."
Club captain Bullen meanwhile was also impressed and glad that Paul Sturrock's men put on a show for the decent Hillsborough crowd.
He said: "A few months ago if you said we would have drawn at home against the league champions I would have taken that. If you had said a few months ago we would have been safe with two games to play then we would have taken that as well. Overall it has been a great couple of months; we have lost one game in eight and according to what the manager said in his notes, that's promotion material so we have proven that we have the players.
"I think the manager and the coaching staff have learnt a lot and so have the players but the end of the season for us has been tremendous. We have got one game to go, it will be a great day out at Derby but I am delighted for all the fans that turned up against Reading because we put a game together that should have beaten the champions."
But Bullen also took time out to praise youngster Tommy Spurr who made an impressive debut and even had to switch to an unaccustomed left-back position to cover for the injured John Hills only five minutes in.
He added: "It's very rare that someone comes in and has a shocking debut to be honest but Tommy's performance was unbelievable for a kid who is only 18 years old. I have played with Tommy many times in the reserves this season because I have been in and out of the first team but I knew that if he did come in that he wouldn't get flustered.
"He came up to me after about 15 minutes and said ‘Bully, I'm knackered' but I think that was more nerves than anything. The kid has done tremendously well but he is not the only one who has impressed because there's two or three who could step in: Sean McAllister came in at Southampton and did tremendously well, Rory McArdle has been out on loan all season and impressed so there is a few hard decisions that the manager has to make this summer regarding these lads.
"These lads can go from strength to strength and Tommy proved that against Reading, and it wasn't a weakened side that they put out either and he has come through with flying colours against a lad who has premiership experience in John Oster."