Do you Salute an officer off post while both are in uniform in South Korea?

There is generally a local Command policy to reference in situations like this. If you are currently in Korea you would be in a better position to answer your own question. While I was on Embassy duty we had local policies that prohibited uniforms to be worn off Embassy/Consulate grounds except when specified otherwise by Command leadership. The exceptions were usually for the Marine Corps Ball or another high visibility event and we would always travel in civilian clothes and change in and out of uniform onsite.

Yes.