Just to answer a few of the questions (I was there last month(US citizen/GCC resident went to Salalah):
1. They do accept the sister countries visas (I know for sure Qatar with the stamp they do, but not 100% sure about Dubai)
2. You can get VOA at Salalah. Supposedly you can also do it online, although I tried like 50 times and it failed.
3. As indicated, the visa is 5OR for tourist visa short term for US Citizens or GCC residents, and 20 OR for longer term. You just pay at the visa desk and then head to the passports.