To get to the city is easy via train (go to Strathfield on the train, and change to city/city circle), If you feel like some shopping, Homebush DFO is a short trip from there (get the 525 bus from Olympic Park Station).
The Olympic Park train doesn't go to Strathfield. You can change at Lidcombe although when special events are on they sometimes have trains running directly to the city. Either way, the train is very easy to use.
You can get a bus, or the ferry into the city too.
Is OP far from the airport and easy to get/to from? Would it be easier/best to head straight to OP from the airport and then go into the CBD for sightseeing on the first day, or just go straight to the CBD, spend the night there, and then travel to OP on the Friday? It's DS's first trip there so I want to show him the sights.
Depending on what time you get in, the train from the airport may be cheaper than a cab. City traffic or queues may make getting a taxi unappealing and the train is very easy to use.
Honestly I'd probably stay in the city, rather than OP. The nightlife in OP is pretty non-existent. There are a few bars and resturants around which might be busier than usual because of the event.
BUT, its certainly feasible to spend the Thursday night in the city only if thats your preference. What's your city accom budget? I personally love the YHA at Circular Quay. You can get harbour views and private rooms and bathrooms and its so cheap.