Online ranks that show next to your name
Music unlocks in conjunction with your save file for MGS4. So if you have a save at the end of the game, you will have all Music Tracks unlocked for MGO, if you have a save after you have beaten a boss, their track will be unlocked. However if you continue a completed save file, you will lose all tracks in MGO until you unlock them again.
Keep spinning your character in circles at the selection screen. When you stop, he will get dizzy and fall over.
When playing in stages where your breath frosts, have Snake play dead while prone. His frosty breath will no longer be seen by others.
During gameplay, in the text menu (on either all or team) type <SONG> for your character to hum/whitsle/sing a part the Metal Gear Online melody.