Hi there!
I asked almost exactly the same question on the Q and A page a few months ago because like you, I enjoy using a diffuser. (This was before I got my GCC.)
I got a bunch of replies, most were of the "do not use" variety, with some saying "pure" essential oils would be safe.
I looked over all of the answers, did a little more research, and eventually decided that it would be better to be safe than sorry. I personally gave up that luxury entirely so that Nico and I wouldn't be worried about his safety. We have the window open (with screen, obviously), a lot of the time to make sure we've got fresh air and the room doesn't smell musty, especially with Nico producing a lot of that particular "bird" scent!
You should probably only use oils if you are very confident that your essential oils would be safe for your bird, or if you know your bird won't be close enough to be affected by them. I think people suggested using the real thing (e.g. using an actual orange instead of using a citrus scented oil, or growing lavender instead of using lavender oil) as an alternative. Keep in mind that no matter what the product, tea tree is toxic to your bird, and also air fresheners are a no-go.