Brett Anderson, Trevor Cahill, Dallas Braden, Gio Gonzalez, and Vin Mazzaro are all 27 or younger, and all five pitchers have ERAs under 4.00 this year. That doesn't tell the whole story, of course, but it shows that the A's have a formidable group of young arms.
There's no reason to expect the A's to change course next year, as long as those five pitchers stay healthy. They should be effective again (xFIP, a fielding-independent stat that evaluates pitchers based on walk, strikeout and grounder rates, has all five starters at 4.33 or below, so they don't appear to be succeeding on luck) and they will all be affordable. Only Braden, who goes to arbitration for the first time, will earn more than $750K in 2011.
Basically every team MLBTR has analyzed so far could pursue starting pitching this winter. The Brewers, Dodgers, D'Backs, Padres and Nationals all seem likely to consider free agent starters this offseason and even the Reds could actively pursue pitching. Last winter, the A's signed Justin Duchscherer and Ben Sheets to high-risk, high-reward deals, but don't expect GM Billy Beane to do the same this winter. The A's have enough starting pitching to get by without aggressive forays into the trade or free agent markets.
The A's have potential starters in the bullpen, on the DL and the minor leagues. Reliever Tyson Ross succeeded as a starter throughout his minor league career, so Oakland could stretch him back into a starting role. Before the season, Baseball America said Ross has "middle-of-the-rotation stuff, and possibly more." Boof Bonser, now in the 'pen, also has experience as a starter, but there's no guarantee that the A's offer Bonser arbitration, since his 2011 salary could be in the $1MM range.
Josh Outman, who started 12 games for the A's last year, will be an option if he recovers from the elbow issues that have sidelined him for most of the season.
If and when the A's encounter injuries, they can also call on minor leaguers for depth. Former first-rounder Clayton Mortensen has posted a 4.14 ERA and struck out 2.4 times as many batters as he has walked at Triple A this year and rotation-mate Travis Banwart has struck out more than a batter per inning. A couple pickups from the Mexican League, Bobby Cramer and Yadel Marti, provide depth and intrigue, if nothing else.
Ross, Outman, Mortensen et al. are not necessarily the sort of starters the A's would choose to rely on, but the A's don't have to rely on them. They have a quintet of effective young starters and can call on their organizational depth when they need it. Beane and the rest of the front office would presumably be open-minded about adding pitching this winter, but they can focus on the lineup or the bullpen, since the A's don't need to add starting pitching.