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."