Oozemon is a tyrant who was sealed away 6,000 DigiYears ago.
According to Seraphimon, Oozemon "ruled the world with a reign of unparalleled terror" until he was imprisoned in a Hyperlock Chamber by Seraphimon, Mervamon, & the "Order of Meridian", who presumably were the precursors to the modern DigiDestined. In addition to being a powerful villain of his own accord, he also has his own batch of enemy foot soldiers & gigantic Digimon.
His hyperlock chamber is soon unearthed by a Tokyo construction crew. Seraphimon sends the DigiDestined to hide the chamber once more, but they are beaten to the scene by Lilithmon & Tactimon. Tactimon releases Oozemon, & asks for Seraphimon's destruction in return. Oozemon enthusiastically agrees, & breaches the Command Center's security with ease. He demolishes the Command Center, & destroys Seraphimon's time warp, causing him to age rapidly. With the destruction of the Command Center, the DigiDestined lose their Digimon, forcing them to go on a quest for a new power source.
Oozemon travels to the moon, & once there easily deposes both Tactimon & Lilithmon, trapping them within a snowglobe. Their henchmen MadLeomon & Oinkmon swear allegiance to Oozemon rather than join Lilithmon & Tactimon in captivity. Oozemon then begina brainwashing the citizens of Tokyo into digging up his Titan-Insectoid Digimon, large insect-like Digimon similar to the DigiDestined's Digimon partners.
After the DigiDestined manage to restore their powers on Phados, they return to Tokyo & battle Oozemon's Digimon with their new Ninja Digimon. Using these Digimon, they are able to destroy one of the Titan-Insectoid Digimon, TyrantKabuterimon. Oozemon combines with the other Titan-Insectoid Digimon, GrandisKuwagamon, & takes on the DigiDestined himself. The battle progresses into outer space, & though Oozemon consistently has the upper hand, he is destroyed when "NinjaOmnimon + Aquilamon" kicks him in the groin & knocks him into the path of Ryan's Comet, which happens to be passing. The comet destroys Oozemon on impact.
"Oozemon + GrandisKuwagamon"
"Oozemon + GrandisKuwagamon" is the DigiXrossed form of Oozemon & GrandisKuwagamon. It has not been officially named.