Now, for the eighth consecutive year, Oswalt will be taking the hill for the Astros when games start counting.
"Any time you can go first, it's nice," Oswalt said. "We still have five guys, and this year, I'm hoping we can get five guys and stick with five guys and have a little competition amongst ourselves."
Oswalt is coming off a great spring start against the Red Sox on Tuesday. But it isn't really the four innings and zero runs that sold Mills -- more like the 137 wins, 3.23 career ERA and three All-Star Game appearances.
"I think everybody kind of understands that, and they kind of thought that anyway," Mills said about Oswalt being the ace of the staff. "All this does, if there was any type of ... haze of any question of this, we just needed to get it out. I'm not going to put any more importance on it than that. ... Just tell [Oswalt] that we appreciate everything that he's done, and he's thrown the ball extremely well, and go from there."
Oswalt, who will get his 10th year started on April 5 at Minute Maid Park for a 6:05 p.m. CT game against the Giants, owns the club record for consecutive Opening Day starts and is tied with Phillies ace Roy Halladay for the lead among active pitchers.
In his previous seven Opening Day starts, the 32-year-old right-hander owns a 3.35 ERA.
In previous years, Oswalt has gone into Spring Training in an open competition for the nod. But this year, despite the fact Wandy Rodriguez was Houston's best starter last year, Oswalt had a pretty good idea he'd be the guy early in camp.
"After we lined it up from the first start, it landed for me on the first day, so I kind of knew it," Oswalt said. "But they didn't tell us for sure. They told us this morning."
Oswalt has notched 15-plus wins and finished in the top five in National League Cy Young Award voting five times in his nine-year career, which has been spent entirely in Houston.
Last year, the Mississippi native finished with a career-low eight wins and a career-high 4.12 ERA. Oswalt said he "never got into a groove where I felt comfortable" in 2009.
But Oswalt has looked good in the small sample size of Spring Training, hurling six shutout innings through two short starts.
"He's thrown the ball so well all spring long," Mills said. "He's been healthy, he's gone about his business the way a No. 1 guy should, and he's throwing the ball like a No. 1 guy should. It's not because of his last outing, but each time he goes out there, it seems to be like almost icing on the cake, because he's throwing the ball really well. And he feels good -- that's important."
With Oswalt starting on Opening Day, Rodriguez -- 14-12 with a 3.02 ERA in 33 starts last year -- will be slotted in as the No. 2 starter, likely followed by Brett Myers, Bud Norris and the winner of the fifth-starter competition between Felipe Paulino and Brian Moehler.
Though Rodriguez acknowledged that starting Opening Day was a goal when he arrived in spring camp, he said he's "happy" being second.
"Every time I'm here, I try to do my best, and then the decision is theirs to see where I'm slotted," Rodriguez said in Spanish. "They decided to start me second. I came with the desire to win the first spot, but they made the decision to give me the second.
"Here, there are lots of good pitchers, and for me to be the second starter, I feel happy."
Alden Gonzalez is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.