Brentford beat Liverpool, Man Utd to Tarkowski deal
Bees manager Mark Warburton admitted the club had been tracking the 21-year-old for some time and expressed his delight at getting the deal over the line.
"James is a very talented young defender who we have monitored for a period of months," Warburton told the club's website. "He is a good footballing centre half, he is comfortable in possession and has a good physique.
"We are delighted we have been able to get him to Brentford on a permanent basis.
"It will give us good choice at centre half and excellent quality in that position.
"James ticks the box for us in terms of his age and he has lots of development left in him.
"We think this is a very good piece of business for the Club.
"James will go back to Oldham for a month when the loan window opens.
"He will come back to us later in the season and is a big part of our plans for the future."