Why is this a problem? Ants outside are virtually never a problem.

The ants aren’t eating your orange tree. They are mostly after the aphids and scale. You can try Pyrethrin. It’s relatively safe. You can also try spraying the leaves with water and Dawn, but it’s time consuming. We live in Florida and have several huge orange trees that produce like crazy. As long as the leaves and blossoms look healthy, I don’t worry too much about ants.

Does your plant have scale insects? Ants are usually attracted to the sweet secretions these insects produce.

You're going to have to kill them

I'd spray it off, hard, then put vaseline in a thick layer all over the trunk. Between the soil and the foliage.

What can i use to get rid of them from my orange tree (in pot, just moved out about a week ago). any ideas?