The former Republic of Ireland centre-half is keen to prove that Cahill's absence from the Wanderers defence will not hinder the team as much as it is feared.
And the 31-year-old defender has already been given the backing by boss Owen Coyle who has named him to plug the gap left by Cahill, starting with the Premier League clash with Aston Villa, following his red card against Arsenal and failed appeal.
"I was aware coming into this season that I maybe wouldn't be first choice but I'd like to think I worked hard," O'Brien told The Bolton News.
"It was unfortunate Gary being sent off but now I have got the opportunity.
"I thought the appeal would have been successful but I guess the FA have to back the decisions their referees make. It was a bit harsh but you have to get on with it.
"One man doesn't make a team and while Gaz (Cahill) is an important part of the squad someone else has to step in now."
O'Brien readily admits he was on the verge of leaving the Reebok Stadium in the summer in a bid to find first-team football.
However the ex-Newcastle United man will be firmly focused on helping Bolton to turn their fortunes around at Villa Park having conceding 13 times in their last three visits.
"Aston Villa away has never been an easy game for us anyway but I'm not expecting it to be anything else," O'Brien added.
"There's no doubting they are a good team. With the change in manager and the timing, it probably unsettled them a little bit.
"But they have got Gerard Houllier in as a manager now and I'm sure he'll be a steadying, calming influence on it. There's no doubt they have got some very good players but that's them - we need to worry about ourselves."
Source: Team Talk
Source: Team Talk