Nearly all of their success has come at the expense of their division rivals -- most notably the newest member.The A's look to sweep the Houston Astros for the second time this month Wednesday when they conclude this three-game set in Oakland.Oakland (11-4) has the second-best record in baseball behind 12-1 Atlanta, an outstanding start for a team many felt couldn't repeat as the AL West champ.The A's have won 10 straight division games against Seattle, Houston (4-10) and Los Angeles after opening the season with back-to-back losses to the Mariners.Oakland has pounded those West foes, hitting .319 while averaging 8.0 runs over the past 10 games. It's outscored the Astros 38-14 in five meetings with the division's new addition.Tuesday's victory was a rare close call for the A's against Houston. They trailed 2-0 after three innings and needed Josh Donaldson's tiebreaking triple in the eighth inning to escape with a 4-3 win."You're not going to score 10 runs every night, so it's nice to win a one-run ballgame," said closer Grant Balfour, who has converted 20 straight save opportunities dating to last season.Donaldson's heroics Tuesday also helped Oakland improve to 10-1 all-time against Houston.The third baseman may make an impact in the series finale since he's batting .364 with a .432 on-base percentage and six runs over the past eight games. He's also driven in five in that stretch, three of which have been game-winners.It would seem the A's may have a tough test with Houston turning to Bud Norris (2-1, 1.96), who is emerging as their top starter after going 7-13 with a 4.65 ERA last year.The right-hander, however, struggled in a 6-3 loss to Oakland on April 6. He surrendered five runs -- two earned -- and six hits, including homers to Jed Lowrie and Coco Crisp, while throwing 122 pitches in 5 2/3 innings.Norris, though, rebounded in tremendous fashion Friday, yielding three hits in seven innings of a 5-0 road win over the Los Angeles Angels."This guy's a No. 1 starter and he's pitched a lot of big games," manager Bo Porter said. "I think his best years are definitely ahead of him. He's growing up right in front of our eyes, and it's a pleasure to see."Porter isn't finding any pleasure in watching his lineup muster a .192 average during a four-game losing streakJason Castro, though, managed to find some success against Bartolo Colon (1-0, 4.15), who beat Norris on April 6. The catcher hit a three-run homer off the right-hander and also singled.Colon lasted six innings in his 2013 debut, which was his first start since being reinstated from a 50-game suspension for a positive testosterone test last August.A three-run homer was again the only damage Colon allowed over seven innings Friday. This one came off the bat of Prince Fielder in a 4-3 win in 12 innings over Detroit.