The 30-year-old left-hander, who won 22 games in 2005, was released by the Philadelphia Phillies on March 16 after trying to make the roster as a reliever. He will attempt to do the same with Baltimore.
A two-time All-Star with the Florida Marlins, Willis has pitched for four teams in nine seasons -- all primarily as a starter. The Phillies signed him to a non-guaranteed contract in December.
Willis, 1-6 with a 5.00 ERA in 13 starts with Cincinnati last season, will report to the Orioles on Wednesday. He has a career record of 72-69 with a 4.17 ERA, but is just 4-15 since 2008.
"The bullpen is a different beast for me as far as in my career, I've never done it before," Willis said, according to MLB.com. "At first it was a little shaky, but I finally started to get my bearings and get comfortable about going out there, because everybody's different in the bullpen -- how to get ready and how to get prepared and stuff like that. I was definitely getting my bearings under my feet, so I'm going to continue to do stuff like that and get used to my role over here."
In three appearances this spring, Willis has allowed five earned runs on five hits and four walks in 2 2/3 innings.
"It seems like the bullpen was already set for Philadelphia, so this is another opportunity to go out there with another young, talented team and go out there and show my stuff," Willis told MLB.com.
Information from The Associated Press was included in this report.