In exchange, the Orioles are getting four minor leaguers -- right-hander Evan Phillips, infielder Jean Carlos Encarnacion, catcher Brett Cumberland and left-hander Bruce Zimmermann -- and international slot money.
Braves manager Brian Snitker applauded four trades made by general manager Alex Anthopoulos. The Braves also added outfielder Adam Duvall, acquired from the Reds late Monday, and relievers Brad Brach and Jonny Venters in the past five days.
"I feel really good,'' Snitker said. "We talked about what you would like to have, and I think he hit all the boxes. Bullpen depth, starting pitcher, maybe a bat off the bench. I think they did a great job."
Gausman, 27, is 5-8 with a 4.43 ERA for the Orioles this season. He is under contract and arbitration-eligible through the 2020 season.
O'Day appeared in 20 games for the Orioles before having season-ending surgery on his left hamstring. He had a 3.60 ERA and two saves before the injury. The 35-year-old veteran has a 2.56 career ERA in 11 seasons and is under contract through 2019.
Cumberland, 23, was selected by the Braves in the second round of the 2016 draft. He is hitting .228 with 11 home runs and 39 RBIs in two levels of the Braves' system this season. He is currently at Double-A Mississippi.
Phillips has an 8.53 ERA in four appearances with the Braves this season. The 23-year-old right-hander is 4-4 with a 1.99 ERA in 31 appearances for Triple-A Gwinnett.
Zimmermann, a 23-year-old left-hander, is 9-4 with a 2.86 ERA in two levels of the Braves' system this season. He also is currently at Double-A Mississippi.
Encarnacion, 20, is hitting .288 with 10 home runs and 57 RBIs for Class-A Rome this season.
"We're always reluctant to move some of our higher-ranked guys,'' Anthopoulos said. "... We like the guys we lost. There are certain guys that are more high-profile. We didn't go into the trade deadline saying we refuse to move on our top prospects. Certain guys are harder to acquire than others. For the right deal for the right player, we're open to it."
The Braves entered Tuesday's trade deadline a half-game behind first-place Philadelphia and five games ahead of third-place Washington in the NL East. The Phillies have also made a push, adding catcher Wilson Ramos and left-hander Aaron Loup on Tuesday after picking up infielder Asdrubal Cabrera last Friday. The Nationals traded away reliever Brandon Kintzler but otherwise stood pat at the deadline.
"We were just trying to make the team better,'' Anthopoulos said. "Obviously it's going to be a tight race in the East. Philadelphia made some nice moves. The Nats, we know how talented they are. ... It's going to be tough. We want to win the division, as all the other teams do, but even the wild card is going to be challenging."
This is the second deal between Atlanta and Baltimore in three days. The Braves acquired right-handed reliever Brach from the O's for international bonus money Sunday.
Also on Tuesday, the Braves put pitcher Peter Moylan on the 10-day disabled list because of a right forearm strain.
The Associated Press contributed to this report.