The 20-year-old will link up with former Toffees Jake Bidwell and Adam Forshaw, who also joined the League One club on loan and have since penned permanent deals.
Bees manager Uwe Rosler told the club website: "We have been working on getting Conor to Griffin Park for eight or nine months.
"We have a good relationship with Everton and have had good feedback on Conor both as a player and a person.
"Conor is a goalscorer. He is a player who can operate deeper if required and score from there and he can play in a 4-3-3 or a 4-4-2.
"He is a very versatile forward and I think he will add goals to our squad. He is a good finisher and doesn't need many chances to score.
"We are very happy to get a player we have known about for a long time and we expect him to have a very successful loan spell, like Jake Bidwell and Adam Forshaw.
"I am looking forward to working with him."