If countries only buy oil in dollars they need to get dollar from somewhere. They need to sell something else and get paid in dollars.

This is and advantage if you have dollars.

Furthermore since the price of both dollars and oils goes up and down all the time, you don't wan to exchange your own currency or sell stuff for dollars at a bad time.

You wan to get dollars while they are cheap and use them later when you can buy oil with those dollars cheaply.

This requires you to keep a certain amount of dollars in reserve.

Dollars that you don't actually use to do anything with them but keep nonetheless available to you if you need it.

This all adds up to people need dollars much more than they would otherwise.

Supply and demand happens and since so many people have a demand for dollars you can add more supply without making it worth less.

It is all much more complicated than that, but it all adds up to a huge advantage for the US and the people in the US who have money.