Ozzie resides in the Middle Ages. One day, Janus appeared in front of him by a Gate created by Lavos. Seeing that he was a human, he sent some Blue Imps at him, which Janus defeated with his magic. Upon seeing the power Janus had, Ozzie likely raised him and taught him the ways of a fiend.
Eventually, Janus became Magus and later became a powerful figure in the Middle Ages era. After Glenn and Cyrus obtain the Masamune, they are confronted by Magus and Ozzie. Ozzie comments that Glenn looks like "a scared little frog with no pond to jump into," so he asks Magus if he can give Glenn a more fitting form. Magus shoots a bolt of lightning at Glenn, which causes him to transform into Frog, making Ozzie partly responsible for Glenn's transformation.
War and Chrono's Arrival
When the fiends were fighting the humans, Ozzie was one of the commanders at Zenan Bridge, When Crono's party arrive at Zenan Bridge during the battle, Ozzie introduces himself, saying he is one of Magus's three generals. He sends skeletons after Crono's party, and when he runs out of them, sends Zombor at them and then flees to the Fiendlord's Keep.
At the Fiendlord's Keep, Ozzie greets them and tells them that Magus was busy and told them that they'll have to do business with him, Slash, and Flea, as well as 100 monsters in the keep. He then disappears.
After defeating Slash and Flea, the party meets Ozzie again. He calls upon Slash and Flea, though they do not appear, resulting in him being "in a pickle" and placing traps ahead of the party, though they pass through all of them. When he is at a dead-end, he tells them that Magus should be done summoning Lavos and then has a barrier surround him while the party defeats him. His barrier is impenetrable, so the party attack some chains that open a trap door below Ozzie, resulting in him falling through the keep.
Sometime later, Ozzie resides at his own castle, named Ozzie's Fort, and now renames himself Great Ozzie (or Ozzie the Great). When Crono's party and Magus arrive there, Ozzie greets them and is surprised to see Magus with Crono's party and calls him a traitor. Ozzie then disappears and sends Flea and Slash at them, both of whom are also surprised to see Magus with Crono's party, though they both are defeated.
As they go through the castle's floors, Ozzie sends various obstacles at them, though the party gets past them all. When he is at a dead-end, he, Slash, and Flea fight the party together, though they are all defeated. Ozzie tells Magus that he used him to create a world for fiends, though Magus tells him that he only sought power. Ozzie then says he cannot lose and has another barrier like the one he had in the Fiendlord's Keep surround him. The party again attacks some chains, though this results in trap doors appearing underneath them. They fall to the floor below and make their way back up to Ozzie.
Once they reach the floor Ozzie is on again, however, a single cat enters the room and hits a switch, resulting in a trap door appearing below Ozzie. He falls through the trap door and is never seen again. However, it is likely he survived because his descendant, Ozzie VIII, lives in Medina village in 1000 A.D.