Luke Scott hit his career-high 26th homer, had two RBIs and scored twice to help the Orioles extend their winning streak to four. Baltimore went 17-11 in August, 17-10 with Showalter in the dugout.
"It's been fun, what can you say? We are getting pitching, we are getting defense and we are getting offense," right fielder Nick Markakis said. "When you get all three of those, good things happen."
The Orioles last had a winning August in 1997 -- the most recent season in which they finished above .500.
"The positive thing is we are doing it against teams that we have struggled with in the past," Markakis said.
Baltimore went 6-0 against the Los Angeles Angels in August and finished the month by beating Boston. The Orioles were 4-24 against those teams last year.
Matusz (7-12) allowed two runs, six hits and a walk in six-plus innings. The left-hander struck out six, including David Ortiz three times.
Koji Uehara, the third Baltimore reliever, retired all five batters he faced to earn his fifth save.
Jed Lowrie homered for the Red Sox, who have dropped three straight. Boston fell eight games behind the AL East-leading New York Yankees and remained seven games behind the second-place Tampa Bay Rays.
"We've got to win games," Lowrie said. "We don't have any other choice."
Lowrie was one of few players to make himself available in the sullen Boston clubhouse. Beckett left by the time reporters were allowed to enter and Ortiz refused to talk.
Beckett (4-4), who came in with a 6.50 ERA, gave up three runs, two earned, and seven hits in seven innings.
"We expect that from Beckett," manager Terry Francona said. "We've come for a long time to lean on him. Tonight was more what he can do."
Baltimore got a first-inning run when Markakis doubled and scored on a single by Scott, who ended a 2-for-21 slump. Markakis became the first player in Orioles history to hit 40 doubles in four successive seasons.
"It's definitely something to be excited about," he said. "You're going to have your struggles, but it's always good to get something like that."
It became 3-0 in the third. With runners on the corners and two outs, Adam Jones hit a grounder in the hole to shortstop Marco Scutaro, who threw wildly to second. Jones was credited with an RBI single, and another run scored while right fielder J.D. Drew retrieved the ball.
"I have nothing to say. I just made a bad throw. That's it," Scutaro said. "I don't have any excuses."
Lowrie ended Baltimore's run of 30 straight scoreless innings in the fifth, hitting his fourth homer following a leadoff single by Mike Lowell. It was only the third home run allowed by Matusz in his last seven starts.
"I made a bad pitch right there, but that's going to happen," Matusz said. "I was able to really come back and bear down and finish it off."
The Red Sox got runners at first and third with two outs in the sixth before Lowell looked at a third strike.
Ortiz is 0-for-9 with eight strikeouts lifetime against Matusz. ... Francona said Boston will reactivate catcher Jarrod Saltalamacchia and infielder/outfielder Eric Patterson from the 15-day DL when rosters expand on Wednesday. ... Jones (bruised shoulder) returned after missing five games and second baseman Brian Roberts (hip strain) returned after missing two games. ... Second baseman Dustin Pedroia (sore left foot) and catcher Jason Varitek (fractured right foot), who are both on the 15-day DL, will be examined on Wednesday by Dr. George Theodore in Baltimore, Francona said. ... The Orioles are 8-1 in series-opening games under Showalter. Before his arrival, they were 9-25.