Put the .ogg files into your player-created mission's folder (generally found in My Documents/Arma 2 Other Profiles/Your name/Missions)

First make a " sound " folder (without Quotes)

Now make a text document. Name it "description.ext"(again with out Quotes)& Save it (Windows by default hides the file extension so it might actually be called Description.ext.txt - which won't be read by the game. You can turn this off by going to Tools - Folder Options and turning the "Hide extensions for known file types" option off.)
Put this in at the begining

class CfgSounds
{
	// List of sounds (.ogg files without the .ogg extension)
	sounds[] = {OFPMainTheme01};

	// Definition for each sound
	class OFPMainTheme01
	{
		name = "OFPMainTheme01.ogg"; // Name for mission editor
		sound[] = {\sound\OFPMainTheme01.ogg, 1, 1.0};
		titles[] = {	};
	};
};

now go in-game create a trigger/waypoint and "on Activation" type playSound "OFPMainTheme01" (WITH quotes this time)

and there you go, now you have OFP music in your mission, enjoy

Operation Flashpoint: Cold War Crisis soundtracks. © Bohemia Interactive Studio