Callahan, a 6-foot-6 offensive lineman from Chaparral High School in Parker, Colorado, redshirted as a freshman in 2012 for the Tigers and appeared in two games for them last fall.
He enrolled at Colorado in July and joined the team for preseason practices that began Saturday.
Colorado submitted a waiver appeal to the NCAA asking that Callahan be allowed to compete immediately based on family reasons instead of having to sit out a year like most transfers do.
The school was informed Monday that the NCAA had ruled in Callahan's favor.
Callahan adds depth to an offensive line that returns just six lettermen.