The Oil Monster resembles a monstrous version of an oil slick. It has a deep purple coloration, 8 yellow eyes, and a red mouth with sharp upper teeth.
In "Sirenix", Tritannus used his toxic powers to create the Oil Monster. It confronted and battled the Winx (only Tecna , Musa, and Flora as Bloom, Aisha, and Stella chased after Tritannus). After a lengthy battle, the Oil Monster was defeated and catapulted far away after being struck by Musa's Reverberating Notes.
The Oil Monster's known form of attack is spewing a stream of toxic sludge out of its mouth; the sludge spit is strong enough to repel Bloom's fire magic and temporarily knock out a fairy.