Have you been to Oman for interview? Or is this offer on the strength of an assessment somewhere in Turkey etc?
Kennytheking has summed this up for you in his first reply.
You need to move on. There is no way you are suing Oman Air.
You had an employment offer at best; and you didn't sign a contract.
They have changed their mind, whether due to you being refused a visa, or a change in hiring requirements, or they found you to be bothersome with your emails. (I have a gut feeling it is the latter, as a wrong word here can finish you off before you've even started. Tact is key.)
As was said earlier, it's best for you to jog on and find a new job!
Farrell