In the event of any injury, it's best to get yourself treated as quickly as possible. If you're on your own, try to get to a medic tent or an ambulance vehicle where a medic might be found. If you're part of a squad and one of your teammates is a medic, call him to you using the Command Menu (STATUS>INJURED) or try to get to him so he can administer field aid. To begin the healing process, get yourself close to the medical unit, be it medic, tent or ambulance, and choose the appropriate option from the Action Menu (for example, HEAL AT MEDIC).
(Source: The OFP Field Manual, Damage & Injury, Infantry).
Sometimes the medic runs right past you and you can never catch him long enough to issue the HEAL AT MEDIC command. OFP player Sphinx has a simple solution to this: shoot the medic in the legs. That ought to slow him down.
When you're commanding a squad, you can also order injured teammates to heal at the medic, tent or ambulance vehicle.
As of OFP 1.20, if a medic himself is injured, he can heal himself.
Oh, a swig of medicinal brandy never hurts.