Hi Nnm. Thanks for the information, it would be good to give the Omani's a castle in their native Arabia, else they'll be in a bit of trouble if the Abbasids come south.
I'm not in charge of the map so I'll let Mirage speak on behalf of that, but your suggestions seem very applicable, simply having the westernmost Oman Province be a castle (if I understand correctly), and the Easternmost be their capital.
I figure one unit we were remiss in not including would be an Omani Pirate unit, which would be logical to have any Omani castle barracks. Idealy, we could make it so only port provinces can recruit them, but unless there's a hidden resource for ports or coastal provinces that we are forgetting, I'd rather not do the work for just that. Nothing special - could use the axe, or if we wanted to be a little innovative, it could be a khandjar and shield wielding unit, with exceptionally fast attack ratings but very very low damage.
However that gives an idea of having a separate Omani unit tier located in their shipwright buildings. Tier 1 would have Omani Pirates, Tier 2 would have the Light naval marines, and tier 2 would have the Medium naval marines. We may want to consider the style we're going for with Oman, if perhaps we want them to recruit their 'auxilia' units (Africans, Tribal Arabs, Baluchis/Iranians/Turks) from castles, and their regional AOR units along with native Omani Units (Marines, Pirates, Swordsmen [though these guys could pass as castle], Ahdath Merchant Militia) in cities. It'd give them an edge made up in the fact that their AOR Could be limited to the south and south-east of the map, or in the fact that their cavalry roster is less strong than their neighbors.