Blues president John Davidson said before the Blues' game against Dallas that Oshie had surgery Saturday morning. The 22-year-old played more than 17 minutes Friday night.
"He cramped up after the game [Friday] night and they figured there could be something to this," Davidson said. "They took him over to the hospital and did a scan and figured out there is an issue.
"We'll have to get him better and back in the lineup."
A first-round pick in 2005, Oshie has a goal and two assists in eight games.
Oshie, who missed 24 games last season with a high ankle sprain, is the second Blues player in as many years to have an appendectomy. Goalie Chris Mason had one early last season.
"The thing about our organization is we're kind of used to it," Davidson said. "It's not a shock to us anymore. When things happen, you try and deal with it, and you go forward."