I was just able to easily book an Oman Airways partner award using Guest miles using their 1-800 call center. Call-in availability mirrored award availability showing on Oman Airways' website. Taxes and fees were 68.80 Omani rials for a flight from Muscat to Bangkok in business class.
The points were instantly transferred from AMEX MR to the Guest program as well. The booking email confirmation from Etihad included both their PNR, and the Oman Airways PNR used to select my seat online.