Music Tracks - 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.

Ranks - Online ranks that show next to your name

Bear Have a large amount of CQC attacks given
Bee Uses lots of scans

Chameleon Prefers Team Sneaking rules
Chicken Rarely Participates in Battle
Crocodile Very good kill death ratio

Doberman A Top-Class Soldier

Eagle Large percents of headshots

Flying Squirrel Have a high percentage of rolling
Fox The Best of the Best
Foxhound A Living Legend

GA-KO Prefers Rescue rules
Grasshopper Stick to crawling the majority of the time

Hound A Superior Soldier

Jaws Large Amount of Knife Kills

Kerotan Prefers Capture rules

Night Owl Use the ENVG a lot

Pigeon Perfers non lethal attacks

Rat Falls for a lot of traps

Sloth Gets shot in the head a lot
Snake Perfers sneaking missions

Tarantula Large amount of Team Kills
Tsuchinoko Rarely Plays
Turtle Use the Cardboard box a lot

Water Bear Have a high survival rate