The Orioles selected Foster's contract from Triple-A Norfolk on Tuesday.
The 26-year-old was acquired from the Mets in the Gregory Soto trade over the summer, so his addition to the 40-man roster was to be expected. Foster pitched in 13 games for Triple-A Norfolk after the trade and had a 3.38 ERA, 1.38 WHIP and 23:8 K:BB over 16 innings.