Parking: Awkward! This is a busy road through Chinnor and there are often other cars already parked. It might be better to drop off a keyboard or similar large instrument, then park your car  elsewhere and walk back.
You can park on the road in Oakley Road or at the carpark next to the Wellington House Surgery (signposted Library, St Andrews School) You can fit up to 3 cars on the drive in front of the church, but all are then blocked in.
This link opens a map which when enlarged names the church, Library and school: https://www.openstreetmap.org/search?query=ox39%204pz#map=17/51.70213/-0.90992