I did OAT's Ultimate Australia back in 2007 (and things could be different now).
FWIW, I remember the OAT guide offering to assist in the purchase of opera tickets for anyone who wanted to see the performance. That way the OAT people got to sit together. In addition, Sydney wasn't until (something like) the 14th day so there was time to the get tickets. Back in 2007 a tour of the opera house was included; however, there was no performance during the tour. The guide also arranged to buy tickets (for those who wanted to do it) to climb the Sydney Harbor Bridge.