As we all know, Arma 2: OA has a bit of an annoying fullscreen. It takes forever to alt-tab from the game and it stops overlays from appearing. Normally, the windowed mode is limited to a small, blurry resolution, but I have found a way to make what's basically borderless window for the game. Simply: Go to your Documents\Arma 2 folder.

Locate "ArmA2OA.cfg"

Change "winH" "winW", "winDefW", & "winDefH" to your monitor's height and width respectively .

For example, here are my settings for 1440p: winW=2560;

winH=1440;

winDefW=2560;

winDefH=1440;

Next, make sure your Render resolution ("Render_W" & "Render_H") also match your monitor:

Render_W=2560;

Render_H=1440;

Finally, change the last line "Windowed" to =1;

Then save the .cfg and Arma 2 OA should launch in the "almost" Borderless Windowed the next time you play!