The Overlord is a useful scouting unit throughout the game, and when upgraded with Ventral Sacs from the Lair or Hive, it can transport ground units, thus serving as the Zerg form of a Dropship. On the negative side it is also a prime target for harass, as unlike the Terran's Supply Depot or the Protoss' Pylon, a stray Overlord is very easy to pick off by a unit like the Corsair or patrolling Marines. It is extremely slow without its speed upgrade, and still only moderately fast with the research. The Overlord is unique in that it is a unit and not a building, but which functions to also serve as the primary means of supply.
The Overlord deals no damage. Beyond various ability upgrades, its armor, or Carapace, can be upgraded at the Evolution Chamber.
Gives the Overlord the standard eight slots of loading. Especially against Protoss, this upgrade is very important, even more so when the front entrance of the base is heavily defended, while the inside is not; a drop of just six or seven Overlords with Hydralisks and Lurkers is deadly mid-game. Since Overlords are so easy to get and necessary to amass otherwise, this gives Zerg perhaps the greatest drop volume ability in the game.
This upgrade is usually forgone until very late in the game, since Overlord speed works better, and Zerg needs to use most of its Vespene Gas on producing units. Increases sight and detection range from 9 to 11.
Increases speed by 400%. This is a fairly important upgrade in games against Terran and Protoss, especially if the Zerg intends on doing a Doom Drop or any other drop-heavy strategy. Also, this allows Overlords more survivability when the Zerg plays against Protoss with roaming Corsairs.
If the map allows for it, Overlords should be kept above cliffs or ridges where the opponent cannot see them with a ground army. They can also just be spread around the map and retreated when a ground army is sighted so the Zerg player knows where the opponent's army is. Overlords can also be placed at potential expansion sites to monitor whether the opponent is taking an expansion. The exception to this advice is Zerg vs. Zerg. (It is a common mistake for Overlords to be sent along with the rest of the Zerg player’s units to a rally point only to be forgotten and picked off later on since it is so slow.)
In Zerg mirror games, the initial Overlord is sent to scout for the opponent's base. Once the base is found the Overlord stays until the Spire starts building. This enables the Zerg player to see whether the opponent is building Drones or Zerglings and adapt accordingly.
Overlords are also sometimes used to soak up bounce damage from attack by Mutalisks. This can allow the Zerg player's Mutalisks to escape damage while they target the opponent's.
The Zerg player must be careful when scouting Terrans with Overlords early game because just one or two Marines will kill it if given the opportunity. However, Overlords can be placed on ridges or in areas where the Terran player cannot snipe them in order to watch for Dropships.
In the mid game and late game, Overlords are often used in Doom Drops, particularly involving melee units such as Ultralisks and Zerglings with a few Defilers with Dark Swarm ability. Because of their mobility, they are also useful in detecting Spider Mines in case the Terran player decides to use Vultures.
As with the other two matchups, the Zerg player usually sends the initial Overlord to the opponent's base for scouting purposes. Some players sacrifice an Overlord to see exactly what Tech branch the Protoss player is going for.
Additionally, some players keep their second Overlord at their natural expansion while it is morphing to watch for a Photon Cannon cheese.
Overlords are generally seen in the mid game as defensive units against Dark Templar harass and as warnings for incoming shuttles. In the late mid game, they can be used as offensive units with the drop technology upgraded. In the late game, Overlords are frequently used for drops, dividing the Protoss player's attention.