Yemen’s geography is similar to Ethiopia’s. Majority of the country is mountainous, fertile and receives a good amount of rainfall.
Oman, with the exclusion of Dhofar, is a barren desert incapable of food self sufficiency. The monsoon season in Dhofar brings around 100-400mm of rainfall which isn’t that much, but enough not to be a desert.
The only advantage Oman has over Yemen is it’s close relationship with Britain and Iran who protect the country, it’s small population and its oil/gas resources.
Ethiopia? okay lol, dunno which region you mean though. Depending on the region, Yemen's got a mix of semi-arid and in between that and Ethiopia. Which Oman also has, it's just mostly secluded to the parts bordering UAE and across Iran. Without oil, Oman would still be doing better because of its tourism (people love mountains in the M.E)