You are required to apply for a Chinese tourist visa(https://www.travelchinaguide.com/embassy/visa/tourist.htm) from Chinese embassy in Oman. The requirements are: original passport valid for at least 6 months, one application form, a recent photo, hotel reservation and round way air tickets and other documents the embassy deems necessary. After you get a visa, you need to obtain a Tibet Travel Permit from Tibet Travel Bureau. And be noticed that individual travelers are not allowed to visit Tibet, you need to join a Tibet Tour(https://www.travelchinaguide.com/tour/tibet/). The China Travel Agency(https://www.travelchinaguide.com/) can handle this permit for you.