We do not know much about the beliefs of Ibadi sect of Oman. If it is the same Ibadi sect which is attributed towards Abdullah ibn Ibadh, their beliefs are found in the books of our elders that are as follows:
(1) It is a branch of Khawarij. They believe that the deeds are parts of belief; hence one who commits major sin remains no longer a momin (believer).
(2) Almighty Allah is the Creator of slaves’ actions.
(3) Ability is before act.
(4) They call Hadhrat Ali (رضى الله تعالى عنه) and his army kafir. If this is the same Ibadi sect or its beliefs are similar, then salah will not be valid behind them.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband