This primary Megadeus is piloted by the main hero of the series, Roger Smith. As the namesake of the series, Big O appears in every episode of the first season. When not in use, it is stored in an underground tunnel below Roger's home. If needed for battle, Big O is summoned by a communicator inside Roger's watch and transported through the tunnels to Roger's location.
One of Big O's most distinguishing features is the piston on each of its arms. These pistons are pressurized so that when they are drawn back and released, they add much more force behind Big O's punches. Roger commonly uses this easily accessible and effective feature to destroy or significantly damage opponents. The only obvious drawback of this ability is that it can only be used in close range.
Big O's arms are also equipped with shields. These are used to protect the Megadeus from damage by blocking enemy attacks.
Arguably its most powerful weapon during the first season, Big O can gather energy into its forehead and fire it as an intense blast. Although very effective, it does take a few seconds to charge up, leaving the Megadeus temporarily vulnerable.
Big O has 3 rows of 3 missiles hidden on each side of its abdominal area for a total of 18 missiles. Roger usually uses these to inflict some heavy damage or to finish an opponent off. The downside is that their quantity is limited, so they must be replenished between battles.
Attached to each of Big O's hips by chains are retractable jetlike anchors. Roger can use these weapons defensively to pull the Megadeus out of harm's way, or offensively by impaling an enemy robot and dragging it closer.
Big O's eyes are able to shoot long-range laser beams. One of Roger's more versatile weapons, this ability can be used to attack enemies at a distance or as a cutting device.
Although Roger rarely uses them, Big O is equipped with machine gun-like cannons hidden in a chest compartment. These are usually used as a last resort in combination with the missiles for extra firepower. However, like the missiles, their ammunition is limited.
Roger can also use Big O's communicator display as a form of radar to pinpoint an enemy's exact location or to gather more information about the enemy. This ability allows him to plan a more precise attack and to avoid dangerous consequences.