Ohlendorf, signed by the Padres on June 4, had a strong relief outing, and the Padres strung together four consecutive singles in a sixth-inning rally to beat the Brewers 5-2.
The game followed a Friday night slugfest when the teams combined for six home runs. None were hit on a sweltering day in front of a capacity crowd at Miller Park.
Ohlendorf (1-0) pitched 4 1/3 innings to earn the victory. He gave up five hits, one earned run and struck out four as the Padres got their sixth road victory this year against 20 losses, the worst such record in the majors.
"Obviously going into this game there was no doubt that Ross was going to pitch," Padres manager Bud Black said. "We talked about it leading up to it. Ross knew that he was going to be the first guy in. He pitched extremely well. He gave us exactly what we hoped for."
Black had such confidence in the way Ohlendorf was pitching that he opted not to pinch-hit for him in the sixth inning with two outs and two runners on base. Ohlendorf struck out, but Black wanted him on the mound in the bottom of the inning.
"I thought he was throwing the ball very well," Black said. "We had a 4-2 lead. I thought getting one more inning out of him was essential based on how our pen was."
Ohlendorf remained in the game until the seventh, when he recorded two quick outs before giving up consecutive singles. He gave way to Luke Gregerson, who retired Rickie Weeks on a line drive to center to end any chance of a Brewers rally.
Huston Street pitched a scoreless ninth to earn his fifth save.
Padres first baseman Yonder Alonso had high praise for Ohlendorf.
"We needed him to step up. He was ready," Alonso said. "You could see it in his eyes early in the game. He knew we were going to ride him. He knows what he's doing. He was pounding the zone and he was working quick. He had a routine and rhythm going early."
Alonso broke out of an 0 for 21 slump with singles in the sixth and seventh innings, one of which drove in a run.
"I think my pitch selection was better today. It's something I've been working on," Alonso said. "The last week or so my pitch selection hasn't been too good.
Mike Fiers (1-2), making his third start of the season for Milwaukee, allowed 10 hits and four earned runs in six innings. He walked one and struck out six.
The Padres got to the 26-year-old in the sixth when Chase Headley, Alonso and Cameron Maybin hit consecutive one-out singles to load the bases. John Baker followed with another single, a shot up the middle, to drive in two runs.
"Fiers threw too many hittable pitches," Brewers manager Ron Roenicke said. "They didn't crush balls but they put the ball in play a lot."
Fiers said it's important that he continue to learn from his mistakes.
"Four runs in six innings is not a great job," he said. "I've got to do better."
Ohlendorf entered the game with one out in the third, replacing Cashner, a 25-year-old making his first start of the season and just the second in a major league career that has spanned parts of three seasons. He threw several pitches that registered 100 mph or greater on the Miller Park scoreboard.
"It's pretty amazing to have a starter throwing that hard," Brewers catcher Martin Maldonado said. "You don't see 100 mph every day."
Cashner, who had made 27 appearances out of the bullpen this season before being moved to the starting rotation, lasted just 2 1/3 innings, throwing 47 pitches. He gave up two hits, one earned run, two walks, threw two wild pitches and recorded five strikeouts.
"I think I threw the ball well," Cashner said. "I thought I could have been a little more efficient. It was a good day. I got some nerves out of the way."
Black said after the game that Cashner would be sent to the Padres AA affiliate in San Antonio to get more work as a starter.
"He's going to start at the AA club to continue to build off of what he did today," Black said. "All in all, he threw the ball fine. Today was a building block for future starts."
Milwaukee took the lead in the first when Norichika Aoki singled with one out. After Ryan Braun struck out, Aoki stole second and advanced to third on a wild pitch. Aramis Ramirez walked and Weeks followed with a double.
The Padres tied it in the third when Everth Cabrera singled, stole second and went to third on a wild pitch. He scored on two-out single by Logan Forsythe. The Brewers went back on top in the fourth when Cody Ransom drew a one-out walk, advanced to second on Nyjer Morgan's bunt hit and scored on a single by Maldonado.
The Padres tied the score in the fifth on Will Venable's RBI double. After the two-run sixth, the Padres added a run in the seventh when Yonder Alonso's two-out single drove in Chris Denorfia, who had a pinch-hit double.
Brewers bench coach Jerry Narron served as third base coach Saturday, replacing Ed Sedar, who left the team temporarily after the death of his father. ... Cashner became the 11th pitcher to start a game for the Padres in 2012, the most since 2009 when the club had a franchise-tying record 15 pitchers start a game. . The Padres and Brewers were the only teams not playing interleague games this weekend. . Venable returned to the lineup after missing four games with a right oblique strain. ... A game after hitting two home runs, Brewers OF Corey Hart went 0 for 5, striking out three times.