The Korean Immigration Office is located at the Passenger Terminal on Osan Air Base. This is where you need to go to get the SOFA stamp or A-3 Visa for your dependents. Make sure you have the proper paperwork filled out and copies, front/back, of all IDs.
The SOFA / Visa office is located inside the Passenger Terminal at Osan Air Base. If you arrive by bus: exit the bus station parking lot turning left down the road. You will see the air field tower ahead. Walk down that road until you get to Jo Rang Mal Road. Turn right at that road and you will be at the Passenger Terminal.
8th Army SOFA / Visa information (just click on the picture): information on this website includes: A-3 Visa / SOFA Application Form 34, Directions for filling out Form 34, and a checklist for obtaining your A-3 Visa or SOFA stamp, and FAQ.