Dermott joined Edmonton this past off-season on a professional tryout during training camp before signing a one-year deal before the start of the season.
The 27-year-old has failed to register a point in 10 games while averaging 13:05 in ice time.
The move comes after the Oilers claimed defenceman Alex Regula off waivers from the Boston Bruins.
Toronto placed Mermis and Alex Nylander on waivers Wednesday in hopes of sending both to the Toronto Marlies. Nylander went unclaimed and was re-assigned.
Mermis has not played a game for the Maple Leafs this season after having jaw surgery after getting hit by a puck during pre-season action. He had three goals and five assists in 47 games with the Minnesota Wild in 2023-24.