"It's a shame," Cubs manager Dale Sveum said of Wood, who threw six scoreless innings. "Once again, he was outstanding."
Matt Guerrier, who joined the Cubs on Wednesday from the Dodgers for Carlos Marmol, took over in the seventh from Wood and walked Derek Norris with two outs. Norris reached third on Seth Smith's single and then scored on passed ball by Castillo, who was catching Guerrier for the first time.
"[Guerrier] made a good pitch, and I think I just didn't read it very good and it went through my legs," Castillo said. "I'm not going to make an excuse. He made a really good pitch and I didn't get a read."
It's believed to be the first time Cubs have lost a game 1-0 on a passed ball since they did so July 1, 1962, to the Braves on an error by catcher Dick Bertell.
Castillo led off the ninth inning against Grant Balfour and struck out swinging. It was clear the Cubs catcher was trying to do something at the plate to make up for the mistake.
"I was a little bit [ticked off], because that doesn't happen often to me," Castillo said of the passed ball. "I was trying to get a good pitch to do something, like a double, or put something in play so we could tie the game or go ahead. It was a tough loss."
Especially for Wood, who held the Athletics to three hits.
"You just have to keep battling," Wood said.
"He's been our All-Star, no question about it," Sveum said of Wood. "He's had some very unfortunate incidents and his earned run average would be quite a bit better than it is without some unfortunate, strange occurrences."
Such as game-winning runs scoring on passed balls.
Wood doesn't have the star power that Opening Day starter Jeff Samardzija does, or the large contract that Edwin Jackson has.
"Sometimes a guy like Travis Wood might get overlooked because of the power of other people," Sveum said. "He's definitely been our All-Star. Every single outing has been pretty impressive."
And Wood hasn't expressed frustration, even though he has had reason to.
"He's dealt with his unfortunate occurrences as well as anybody," Sveum said. "He holds himself very accountable. One bad pitch, he ends up giving up two or three runs in a seven-inning game, and he holds himself accountable. The good thing about Travis is he's the same guy every day he shows up."
On Thursday, Wood sounded frustrated that he hadn't done more.
"I've got to figure out how to win the ballgames," Wood said. "I've been able to keep them off-balance and not score, but I have to help our team pull through and win the games."
Wood's 0-3 with a 2.58 ERA in his last six starts, and he's left the game with the lead just once. There's not much more he can do.
"As far as today, I didn't give up a run, but other starts, I've had them right there," Wood said. "I have to find a way to step it up more."
One thing the Cubs did learn in their first series in Oakland is not to run on left fielder Yoenis Cespedes. Starlin Castro was at second with one out in the fourth and tried to score on Alfonso Soriano's single, but Cespedes threw him out at home.
"I thought he was [going to score]," Sveum said. "It seemed like a no-brainer right off the bat. I don't know what happened there."
"It surprised me," Castro said. "I ran hard right away. When I saw the ball hit, I went right away to home plate and he made a good throw."
Castro had never seen Cespedes in the outfield.
"The guy made a great throw," Castro said. "I thought I'd score easy. I never stopped, I never said to myself, 'I'm out.' I ran hard right away [from] second base. I was surprised I was out."
"It was a perfect throw," Norris said. "He gave me a long hop right to my stomach. It was close, but he's got such a great arm and is a great athlete that he made a terrific throw."
There was a bizarre sequence in the eighth when home plate umpire Brian Gorman halted play during Pedro Strop's pitch to Jed Lowrie. Lowrie appeared to ground out to first, but was told that he wasn't out and Gorman called Strop for an "illegal pitch." That prompted an argument from Sveum.
"[Gorman] said there was an illegal pitch and the hitter wasn't ready, but [Lowrie] had both feet in the batter's box," Sveum said. "I know by the rule that constitutes you're ready to hit."
The Cubs weren't able to, though, against Oakland starter Dan Straily, who was promoted from Triple-A Sacramento for the start. The right-hander threw seven scoreless innings, striking out six. Ryan Cook pitched the eighth and Balfour the ninth, picking up his 21st save.
Despite the loss, the Cubs finished with a winning road trip (5-4) and will have a winning record in Interleague Play (11-6) for the first time since 2007. At least they proved they could win west of the Mississippi River. Last season, the Cubs were 1-18 on the road against National League West teams, picking up their only win in the last game.