Tampa Bay opened the scoring on Miller's two-run single off Baltimore's Dale Thayer in the top of the third, and Casali drove in former Oriole Steve Pearce in the fourth.
After Chris Davis' RBI groundout scored Manny Machado to make it 3-1 in the bottom of the fourth, Kiermaier's RBI triple off Todd Redmond helped the Rays pull away with a six-run fifth.
Redmond was charged with four runs, while O's Minor League pitcher Jon Keller allowed three and didn't record an out.
Machado and Adam Jones picked up RBIs during a four-run fifth for the Orioles.
Gausman faced the minimum over two innings in his first spring start. He gave up one hit, a first-inning single from Corey Dickerson, who was thrown out by catcher Matt Wieters trying to steal second base.
"I actually came down and threw a couple more [pitches] just to get me to 40, better myself for the next outing," Gausman said of his 31-pitch debut. "Obviously happy to get through two innings and get some guys some ground-ball work. It's a little weird facing Pearce. You see him play every day for the last three years, and seeing him in the box was a little bit weird. You realize how much on the plate he actually is. It was kind of cool, though."
Rays starting pitcher Drew Smyly didn't allow a hit over his two innings, striking out three in the 30-pitch performance.
"Felt great, just a lot of fun, felt awesome to be back out there competing against a really good lineup," Smyly said. "Get your feet wet, back on the mound for the first time. It was just a lot of fun for me."
Up next for Rays: Jake Odorizzi will make his second start of the spring on Sunday when the Rays host the Twins at 1:05 p.m. ET at Charlotte Sports Park, live on Gameday Audio. Also expected to see action are right-handers Ryan Webb, Brad Boxberger, Danny Farquhar, and Eddie Gamboa and left-handers Dana Eveland and Blake Snell. Kyle Gibson will start for the Twins.
Up next for Orioles: The Orioles visit the rival Red Sox on Sunday at 1:05 p.m. ET, live on MLB.TV and Gameday Audio. Right-hander Mike Wright, making his second Spring Training appearance, will start opposite Boston's Clay Buchholz.
Brittany Ghiroli is a reporter for MLB.com. Read her blog, Britt's Bird Watch, follow her on Facebook and Twitter @britt_ghiroli, and listen to her podcast. This story was not subject to the approval of Major League Baseball or its clubs.