Welcome to the joys of GTV ownership!
By the O2 sensor he probably means the Lambda sensor. It's screwed into the exahust just after the downpipe, and measures the amount of oxygen in the exhaust. Helps the ECU to manage the fuel settings to minimise emissions.
He'll have reset the ECU so that it's forgotten about the Lambda problem - but it might come up again. Your Lambda sensor could be out of whack a little and not set off the warning light. If this is the case you'll probably find poor running at low revs, a slightly smoky and smelly exhaust, and (in the end) an MoT failure on emissions.
If you're in North London I'd recommend booking a free Saturday surgery appointment at Autolusso ... Ned & the guys will give your new pride and joy a once-over and give you a list of anything they'd recommend you have dealt with. They don't over-recommend work - like many others on here, I've found that they've saved me money rather than fleeced me!