What are all these coordinates being shouted out and displayed in messages?

FYI, the following text is taken from file help file README.CHM, in your OFP install directory:

You will often receive orders which locate targets using a realistic military reference system consisting of two numbers. The first indicates relative angle, the second relative distance in meters. Both numbers refer to "tens" of units - in order to convert them to standard units, you will need to add a zero to each.

Note that the first number of these military reference coordinates is relative to a zero compass bearing (i.e., north). As an example, an order to go to 27 10 would mean bear 270 degrees (which, in this example, is due west) and move a distance of 100 meters in that direction.

This is not to be confused with the clock positions shouted out by your commander. For example, "RPG SOLDIER, 9 O'CLOCK" is relative to the direction your commander is facing - not your direction. To help you out, there's a small clock display that pops up in the lower left corner of the screen.

If you're wondering why the clock positions are relative to the commander's position and not to the players, well, that's how it would be in the battlefield, too. It's your responsibility to spot your commander and determine the direction he's facing. If you can't well, that's what the clock display is for, isn't it?!

In addition to these numeric reference coordinates, there are map coordinates, too, consisting of two letters and two digits. If you look at your map (press the "M" key), you'll see that the map is divided into squares. Each square can be referenced by these coordinates, which are shown across the top of the map and down along the map's left side.