Birmingham manager Steve Bruce has signed his son Alex from Blackburn on a two-and-a-half-year deal.
Bruce, a central defender or midfielder, is on a three-month loan at Coca-Cola League One Oldham which expires in March.
The 20-year-old has made eight appearances for the Latics this season.
He converted the decisive penalty in the LDV Vans Trophy area semi-final against Tranmere this week.