So I just got a new computer + keyboard/mouse.
I redownloaded/installed ARMA3 through my Steam account.
I loaded up the game and made a little scenario in EDEN editor, but when I was in control of my character, WASD, inventory, crouch prone and any other buttons didn't work. My character would also randomly move, and it was like my buttons were shuffled.
I already went through the in-game settings, but it seems like the game is constantly processing my keyboard pressing the O and I think ALT button.
I found that if I hold down right ALT, it seems to work perfectly fine. WASD will work, inventory crouch and prone buttons too, but letting it go just breaks the controls again.