How much a average salary in oman?

Hello- the rate conversion is about 1 rials to 2.6 usd.   It would clearly depend on whether you're arriving alone or with a family.  Also, your lifestyle....

Yes,Im alone

have job offer with $750 usd , with accommodation ,food,

in a 5 star hotel as a fitness trainer,
is it good?

Well Sevda,  750 USD would convert to less than 300 OMR. Even with accommodation and food provided, its a below average salary.
You are probably aware there is also restriction on expat female visas currently. Eating out in a decent non-veg restaurant for a single person will roughly  be 20 USD.
Taking a taxi (non-shared)  even for a short distance is about 8USD.
A moderate salary with food and accommodation
provided will be 1300USD.
Since you are a fitness trainer, you could possibly look at taking private classes, otherwise I am afraid the salary is just not good enough.
You should also be considering your salary in your current job while you are evaluating this offer. If you are drawing lesser than this, I guess you would then be the best person to decide.
Yes, as a country, this is a good peaceful place to live in with a great work - life balance.

Hope this helps you take a decision.
Cheers

Yea...

I've never been in Oman,so I dont know about Life there...
so,I can offer my salary to them,then if they agree I will accept...



Thank you so much 

Sveda what exactly is your profession and what is your ultimate dream. From your posts it is not clear. You look confused

Sveda what exactly is your profession and what is your ultimate dream. From your posts it is not clear. You look confused

well,I have offer from Oman as a fitness Instructor ,and I accepted it,
and Im not confused ,im just waiting for my Visa