Blades boss Chris Wilder confirmed on Monday that the north Wales club had made a move for the attacking midfielder.
Wilder had accused Wrexham of adopting a “blanket bombing” approach to the final stages of the summer window.
“This is not a criticism, but Wrexham are in for half the division,” Wilder said in his press conference ahead of his side’s home Championship fixture with Bolton Wanderers on Tuesday (19:45 BST).
O’Hare’s arrival is a welcome boost to Phil Parkinson whose side take on Millwall at The Den on Wednesday night (19:45 BST).
Discussing the capture of O’Hare, Parkinson said: “We’re delighted to welcome Callum to the club.
“He’s a player with a lot of Championship experience, and he’s going to be a great addition for us for the coming seasons.”