Whether you're playing an SP or campaign mission as a grunt or as a commander, you might get lost or your teammates might get separated from you and not know where you are.
If you yourself are lost, there are several things that can help you. First of all, if you're playing OFP in cadet mode, your position is noted on the mission map screen by a black crosshair.
But let's assume you're playing in veteran mode. If you're not commanding any troops, you're going to have to figure out your position the hard way. Look around you for geographical landmarks, such as high mountains, large forests or towns and crossroads and try to match these up with a location on the map. Don't forget to use your compass to help you out with directions.
If you're commanding a team, you can easily figure out your location by selecting one or more teammates and left-clicking on the ground anywhere near you to get them to move right there. Your coordinates will be announced to them and they'll come running.