score:6
Well. That was fairly painless.
ONTC does not provide a service between Abu Dhabi and Muscat. From Dubai to Muscat, buses leave at 7:30, 15:30 and 23:00, arriving at, respectively, 13:40, 21:35 and 5:00 (the next day).
The price is quoted at 5.500, which probably means 5.5, which is about 11 euros, one way.
I got this from the horse's mouth, ONTC responded to my email query within 15 minutes.
They tell me the bus departs from the ONTC office, which is in Dubai Dunta car park (0097104042525909), though I can't find that on Google Maps.