The oil is probably coming from the rear main oil seal on the crankshaft. It's relatively common and shouldn't be of great concern providing it's not a large pool of oil. Check the engine oil level regularly and if you see it dropping rather quickly have a mechanic check the leak. There are anti-rattle washers in the brakes and they are sometimes lost or not replaced when the brakes are serviced, so have a brake specialist check them for you.

One flat battery is unfortunate, two is a worry, but five should have been a clear sign something was wrong and that your car needed to be checked by a mechanic. Had you acted while the warranty was in effect you could have had it fixed at no cost to you.

I assume the pads haven't been changed yet. Take it to the dealer and ask him to treat it with anti-squeal lubricant, which should fix the problem. If the pads have been changed, check with the mechanic who did the job and find out which pads were used. If he didn't use factory pads, perhaps you should have the pads replaced with the Holden-recommended ones.