Roger summons the Megadeus by calling its name into his wristwatch, and declaring it's "showtime".
At times, Big O appears to have a mind of its own. When Roger is in danger of being killed in "R.D.", the megadeus goes to its master and stops the assassin, who questions Roger about who controls it. During the fight with Bonaparte in "The Third Big", Big O locks itself and lets Roger out because R. Dorothy is in danger. In "Enemy Is Another Big!", Schwarzwald questions if the Bigs truly need pilots.
Instead of blocky body parts, Sato designed a mecha where the chest and hips were not separate from the rest of the body. Big O is slow, lumbering, and heavy compared to more agile and lighter mecha featured in other anime series. That way, movement would seem sluggish, comparable to the giants of old tokusatsu shows. According to Sato, the name "Big O" comes from the opening theme of "Daitetsujin 17", "Oh! Giant Ironman".
The megadeuses are portrayed as semi-sentient beings, capable of acting on their own. Unlike the mechas of other mecha/robot anime, the megadeuses do not exhibit a ninja-like agility. What they lack in agility, they offset with sheer power. The Megadeus's are heavily armed with weaponry like missiles, blade weapons, machine guns, and lasers.
Unlike other mecha series, where the robot's weapons come from specifically designed equipment or spiritual energy, the creators of "The Big O" used older technologies as the starting point.
The use of "Cast in the name of God, Ye not guilty" is a design of series director Kazuyoshi Katayama. Katayama first learned of the phrase through a magazine article on John Milius and the "Conan the Barbarian" series. According to the article, the phrase comes from the swords of executioners in the 17th century. A variation of the phrase ("Suffer no guilt ye who wields this in the name of Crom") is engraved in Conan's sword. Katayama liked the sound of it and decided to use it as the call to arms.
The nature of the megadeus is tied to the event 40 years prior. Several of Roger's dreams suggest that Big O and several other Big-type megadeuses played a part in the cataclysm that destroyed the world. His dreams and nightmares show the Big models attacking the city and marching in formation into battle, while a winged giant oversees the onslaught. Other flashbacks show Roger in a military uniform while piloting Big O. ("Hydra")
Despite the lack of speed, it is at least capable of limited running. Movement speed is more or less directly related to the pilot's own reaction time.
The cockpit is located in the neck area. The actual pilot console is enclosed in a glass dome on a raised pedestal to protect the pilot from harsh environments, and Roger is able to see through the large orange viewport on Big O's neck. When entering or exiting the megadeus, the entire collar portion rises up on hydraulic pistons.
It has a radio and videophone for communication. The Griffon, Roger's car, can be stored in either foot. ("Stripes")
Big O has also demonstrated the ability to open the mouthpiece on its faceplate. ("The Show Must Go On")
Big O can link remotely with Roger's wristwatch even if Big O is not piloted by Roger Smith and use it to transmit images recorded by the watch. ("The Third Big")
The material that makes up Big O's head ornament (crown) and cockpit viewport can deflect energy beams ("Beck Comes Back") but it can also crack under pressure ("Enemy Is Another Big!") and even be destroyed ("The Show Must Go On"). It seems to be made of a special tinted, light refracting polycrystalline ceramic (possibly silicon nitride). The rest of Big O's structure is covered in heavy armor plating, reflecting its function as a land unit required to survive heavy combat. The heaviest armor is on the arms for charging attacks and shielding Big O's torso.
The Big O possesses a varied array of weapons. The cockpit includes an assortment of pedals, levers and buttons that control the Megadeus. The weapons are hidden within a Megadeus, so only by displacing certain parts of the armor can the pilot make use of them.
Laser beams shot out of the eyes. First seen in "Bring Back My Ghost". They have a long range and can be used for a cutting device. An alternate arc line was fired from the headband in "Enemy Is Another Big!".
The upper chest houses 6 machine gun barrels. First seen in "Enemy Is Another Big!" They are a combination of the machine gun and an autocannon. The ammunition must be replaced after use.
First used in "Electric City". It is a concentrated energy beam fired from Big O's crown. The weapon is charged when Big O points its forearms upward and continues charging until Big O slams its fists together in front of it, at which point a beam fires continuously until the hands separate. While charging the weapon, the Megadeus is temporarily vulnerable. The beam can be sustained for a long time ("Day of the Advent") or fired in short continuous bursts ("Hydra"), though the output grows weaker the longer it is sustained.
Used during "The Show Must Go On". Big O's inside and outside are transformed and a massive cannon emerges from the chest. The Mobydick Anchors are deployed to withstand the gun's recoil and the Plasma Gimmick is activated to protect the momentarily incapacitated megadeus during the Final Stage. Final Stage is started by R. Dorothy Wayneright. The cannon has enough destructive force to destroy an entire dome in one shot and is discarded immediately after firing. For unknown reasons (perhaps to "shape" the cannon blast and increase its power), the Sudden Impact pistons in both arms are fired straight ahead immediately before the cannon fires.
18 missiles in the abdominal area in 3 rows of 3 missiles per side. First seen in "Underground Terror". They must be replenished after firing.
Rocket-jet propelled anchor chain combinations in the hip/thigh area. During "Roger the Wanderer", it is revealed the waist is actually made up by 2 rows of 8 anchors each. The anchors may be guided from the cockpit, as seen in "Twisted Memories". Also, it appears as though there are multiple anchors and chains in each launcher. This is seen on several occasions when Big O has fired off an anchor, detached the chain, and another anchor appears in it its place on Big O's hip. ("Stripes") They are used to pull the Big O out of danger or to impale an enemy and drag it closer. Another use is that they can stabilize the Big O. The name refers to the novel "Moby-Dick; or, The Whale".
A pair of multiple barrel firearm machine guns contained within the forearms, first used during "Negotiations with the Dead". Once activated, the armor covering the forearm opens exposing 4 gun barrels. The rotating barrels fire energy volleys at high-speed, while generating a ring of energy that explodes on impact.
First seen in "Hydra". Roger activates the weapon by stepping on a button in the cockpit's floor. The armor on Big O's shoulders and forearms opens, revealing static shield generators. Roger utilizes the generated energy to decimate the Hydra. During Big O's fight with Big Fau, the latter utilizes some kind of energy shield, which may be its own Plasma Gimmick.
It works by retracting the pressurized pistons inside the arms and subsequently releasing them. These pistons are pressurized so that when they are drawn back and released, they add much more force behind Big O's punches. The only drawback of this ability is that it can only be used in close range. In "Roger the Negotiator", the combined strength of Big O's punch and the Sudden Impact creates a hole in Dorothy-1's chest. In "Beck Comes Back", the released gust beheads the Beck Victory Deluxe. In "Enemy Is Another Big!", it goes through 3 skyscrapers. The Sudden Impact can also be used to propel Big O through water and is a key component in the Final Stage.