The 21-year-old left-hander struck out a career-high 12 over six one-hit innings as the Stockton Ports rolled to a 4-1 victory over the Visalia Rawhide.
"It was fun," Hornbeck said. "I was trying to go out there and give the team a chance to win. I've been pitching well lately, but it was a pretty good night for me."
Hornbeck (1-0), Oakland's 32nd-round pick in the 2008 Draft, was used exclusively as a reliever during his first professional season, going 3-2 with a 4.58 ERA in 21 appearances between short-season Vancouver and Kane County.
The 6-foot-5, 180-pound Phoenix native didn't have to wait long to show off his stuff as a starter with the Cougars this year. He fired five hitless innings against Burlington in his season debut on April 15 and five one-hit frames six days later against the Bees.
"The biggest difference [between starting and relieving] has been my ability to throw more strikes and tone down my delivery," Hornbeck said. "I'm not trying to put everything into every pitch."
In seven appearances, including four starts, with Kane County, he went 4-0 with a 1.24 ERA, striking out 10 over seven three-hit innings against Quad Cities in his final Midwest League start on May 17.
"Ever since Spring Training I've been working on my command and working both sides of the plate with my fastball," Hornbeck said. "My pitching coach at Kane County, Jimmy Escalante, was really hands-on with me, helping me keep my fastball down in the zone to get some ground balls."
Hornbeck carried that success into Banner Island Ballpark on Friday, retiring the first nine batters before surrendering a leadoff double to Ollie Linton Jr. in the fourth.
The Kansas State product rebounded quickly, getting Elijah Rumler on a fly ball to left field before striking out Ryne White and Collin Cowgill to escape his only legitimate trouble of the night.
After walking Konrad Schmidt to start the sixth, Hornbeck finished up strong, fanning Derrick Walker, Linton and Rumler before walking off to cheers from the partisan crowd of 4,214.
"They could have put me back out there, but they didn't want me to go over 100 pitches," said Hornbeck, who shut it down after 88 pitches, despite fanning eight of the final nine batters he faced.
Lance Sewell and Justin Friend combined to yield a run on three hits over the final three frames, wrapping up Hornbeck's first Cal League win.
"I just want to keep on pitching good and developing all my pitches," Hornbeck said. "I'm just happy the A's gave me an opportunity to move up."
Larry Cobb backed Hornbeck offensively, going 2-for-3 with a homer and two RBIs for the Ports, who have won six of their last nine games.
John Torenli is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.