The backup infielder sprinted around the bases after homering in his first AL at-bat, added a go-ahead single and finished with four RBIs Sunday in a 9-4 victory over the Los Angeles Angels.
"I had a lot of energy. I almost had to calm myself down and take it down a couple of notches. It's the little kid in me," Rosales said.
Rosales spent portions of the previous two seasons with Cincinnati before being dealt to the A's in February.
"At first, I was shocked and heartbroken when I got traded. My mind was set on making my way up through the Reds' organization and sticking there. But these guys really welcomed me to the club and I'm enjoying it here," he said.
The A's, who finished 22 games behind the Angels in the AL West last season, have started off 5-2.
The Angels went 2-5 on their season-opening homestand. They begin a road trip on Tuesday at New York, where the Yankees will receive their World Series rings. So will Angels new DH Hideki Matsui, the Series MVP, who spent his seven previous big league seasons in the Bronx. The Yankees beat the Angels in the AL championship series.
"You're always going to watch something like that with interest. That's what all of us are playing for," Angels manager Mike Scioscia said. "I guess we're a little jealous because they beat us last year and went on to win the World Series. But I don't think we need to see a ring ceremony to understand the magnitude of it and what we're playing for, and to be motivated to get to that final step."
Rosales hit a solo homer in the second inning into the front row of the seats in the left-field corner. That was Oakland's only hit through the first six innings against Joe Saunders.
Saunders (0-2) retired 13 in a row before giving up three straight hits to start a three-run seventh. Rosales capped the rally with a two-run single. He also drew a bases-loaded walk in the eighth as the A's pulled away.
Dallas Braden (1-0) allowed three runs and five hits over six innings. In his season debut last Tuesday against Seattle, the left-hander struck a career-high 10 in seven innings.
Rookie Tyson Ross pitched three innings for his first major league save.
"We feel like we can compete with anybody, and we stack up well against the entire division," Braden said. "It's a great way to start the season. We're just full of confidence right now. I knew today was going to be a day that we were going to have guys in there who were going to be anxious and ready, and I knew it was going to pay off if I could keep us in the game."
Rosales hit four home runs in 259 at-bats over the previous two seasons with the Reds.
"I thought it was pretty obvious what my role was going to be here," Rosales said. "But the opportunity and the experience I had last year I think has helped me know what my role is this year and know how to handle it."
The A's tacked on three more runs in the eighth, all unearned because of third baseman Brandon Wood's throwing error. Wood, a perennial prospect who has taken over as the Angels' regular third baseman after the departure of free agent Chone Figgins, is 1 for 19 with seven strikeouts through his first five games. The error was his first of the season.
Bobby Abreu hit a solo home run in the Angels' first. The two-time All-Star snapped a drought of 25 at-bats without an RBI, the longest of his 15-year career to start a season.
Angels pitchers have allowed at least one homer in each of their first seven games for the first time in the franchise's 50-year history. The previous longest streak from the start of a season was five, in 1969 and 1981. ... Juan Rivera homered in the ninth for Los Angeles, which has been outscored 29-9 in its last three losses. ... Braden was 0-3 with a 6.35 ERA in his three starts against the Angels in 2009.