I have OA in my left knee, so this might help. prior to what's below, my left knee felt normal
About 3 years ago, I stepped out of my shower and didn't have my full weight on it when I heard/felt a loud POP. I applied RICE (rest it, ice it, etc) immediately and should of called my doctor (I called him a week later). my knee cap could move 1/2" sideways, had muscle spasms, my knee swelled and couldn't move it....and it hurt badly but not the worse I've felt.