If you're in need of a car on your trip, HOJ is a good place to get one. HOJ has 24 hour service so it's also good for emergencies (e.g. accidents, last minute). It's a locally owned and operated company. Address: 1510 Speers Road (at Third Line), Oakville, Ontario. Directions are on the website.
The transit system in Oakville is very frustrating for a number of reasons. The buses run infrequently on seemingly erratic schedules, hours of operation are poor (only in recent years did they begin services on Sundays), and it often takes a long time to get somewhere. But, it's there if you need it, just make sure to plan your trip in advance so you're not stuck waiting for a bus that isn't coming for an hour.
The buses meet at the GO station on Cross Avenue. This is the transport hub in Oakville. Trains (VIA, GO) and intercity buses also meet here.
If you're planning a day trip to Toronto from Oakville, I wouldn't bother with the QEW. It's usually bumper-to-bumper and will test your patience. Take Go Transit instead. You will get downtown in under an hour in a relaxed setting. The train drops you off right at Union station, where you can catch the TTC to most points in the city.