The deal could be completed as soon as Monday, predicated on Ohman passing his physical, the source said.
In 63 appearances last season, the left-handed Ohman had a 3.21 ERA with 18 holds and 43 strikeouts in 42 innings.
When Ohman signs, it will mark his second stint in Chicago. Originally drafted by the Cubs in the 1998, Ohman spent parts of five seasons on the North Side.
Bruce Levine covers baseball for ESPNChicago.com.