Gallagher has a 5.73 ERA and one save over 11 innings with the Cubs this season. He is up with Chicago for the third time this year.
He also has played for Double-A Tennessee and Iowa, and has a 10-3 combined record with a 3.03 ERA in 16 starts.
Cubs manager Lou Piniella says the move was made to give the team more innings in the bullpen.
Piniella said Gallagher may also get an occasional spot start.
Ohman was 1-4 with a 5.06 ERA for Chicago this season. He has a 4.34 ERA in 211 career relief appearances.