One Man Army (sometimes abbreviated as "OMA") is a Tier 1 perk available in the multiplayer of Call of Duty: Modern Warfare 2. It allows the user to switch classes without needing to respawn. However, as a downside, it removes the user's secondary weapon. The perk is unlocked at level 45.
There is a five second waiting period during which time the player can only move, crouch, jump and prone. When the player switches from their primary weapon to the backpack, it brings up the class menu. From there the player can pick the class desired to switch to. Users can switch to the same class repeatedly, or through several different classes with One Man Army.

One Man Army Pro is unlocked after a player has received 120 kills while using the One Man Army perk. One Man Army Pro reduces the time required to perform the class change to three seconds.

Although many players use the perk to resupply ammo for weapons, especially Grenade Launcher grenades, One Man Army was intended for more tactical gameplay. For example, one could have several OMA classes for different tactics. For example, one could switch to a Cold-Blooded class when the enemy has air support up, or Hardline to acquire a killstreak early, or Danger Close Pro to increase killstreak damage. A player could effortlessly switch to any of these classes for almost any different weapon.

OMA is often used with an assault rifle with an underslung Grenade Launcher, as well as Danger Close, to have an infinite supply of extremely lethal Grenade Launcher rounds. Danger Close would increase the blast radius of the grenades, and OMA would resupply the rounds. The setup is popular, but is frowned upon by most of the community, as many see the combination as overpowered.

*for newbies*
You all have to decide and choose your side. Be with campers & tubers or join sucks hood and fight against campurs. You decide. But better choose cuz we have limitless powers!!11

ONE MAN ARMY