Opposing Force is a "DLC" to my missionpack, The Red Eagles.
In Opposing Force, you play as Sean, a corporal of the army stationed in Area 69. In case you haven't played my missionpack, in The Red Eagles you're fight AGAINST the army from the perspective of a member of the Red Eagles gang.

In Opposing Force, you'll be helping Colonel Mills to defeat the Red Eagles, a gang which has been an increasing problem to the areas of Tierra Robada and Bone County.

Also, I suggest you to play at least the first chapter of my missionpack, The Red Eagles, since this "DLC" contains spoilers to it (and vice-versa).