Metal Gear Online, abbreviated MGO, is a PlayStation3 exclusive online multiplayer spin-off of the Metal Gear video game series.MGO allows up to 16-players to engage in online tactical warfare. Players create a character by choosing their name, gender (only after purchasing the GENE Expansion), race and voice. Creating a new character requires the old to be deleted or an additional character slot to be purchased.

A character's level, ranging from 0 to 22, and grade, ranging from Rookie to S+, indicate battle prowess. Level is affected by experience points gained or lost at the end of all Statistics enabled rounds, while grade is only controlled by the results of Survival and Tournament rounds. Statistics are recorded for each character, as well as a Title, Awards and Match history. Based on these weekly and long term statistics, characters receive Animal Titles that denote their in-game behavior. For instance, frequently injecting enemies with the Scanning Plug (colloquially known as 'stinging') will lead to the Bee rank, while using ENVG (Enhanced Night Vision Goggles) for a specific share of weekly play time grants the Night Owl rank. An inner rank hierarchy chooses which one of the acquired ranks will be active.

Characters can enhance their abilities by equipping skills, register skill sets and load them automatically or manually before a match's start. Skills range from the straight-forward, like Assault Rifle+ that decreases reload time and recoil, to more complex, like Mοnomania which essentially turns bullets into short-term tracking devices. Equipped skills level up when pertinent actions are performed by, or at, the character. Depending on their level, skills occupy from 1 to the maximum of 4 available skill slots.

Clans, mutually exclusive groups of up to 64 characters, are also supported, uniting members under the clan name, emblem and reputation. The clan's leader chooses its emblem, decides over the inclusion of clan applicants, and needs to be at least level 3, with over 20 hours online play.

Players can enter Training lobbies and practice the game's controls against training dolls or receive instruction from other players. Players who graduate from an instructor course and have at least 20 hours of play can gain the Instructor skill.