Look under Settings, use scenario attributes, there you can assign up to two different types of OBA for each side, you also must equip your leader units with radios under the support weapons, and I believe each OBA needs its own radio, although I am not positive of that due to not using OBA that often.
I believe whq is correct. Also, the AI won't wait for a radio, if you have the radio and leader come on a later turn. Ai will use it asap with an "offboard" spotter to boot!