I have to disagree with mini when he writes that you want to be as close to home plate as possible.
The vantage point at most MLB stadiums is considerably poorer as one gets closer to the field making it more difficult to see the ball especially during day game when the background has plent of white shirts.
For Oakland, seats with a great view and good value would be the Plaza Infield Section, rows 1-8. Try to get seats within the boundries of 1st and 3rd base, or sections 213-222
Some people may argue, "what about the Field Infield section?" Those are good but if you try to get a "front row" in that section you are actually getting row 21 which has a walkway right in front of you, feeding the section below you and above you. This translates to a whole bunch of people walking in front of you during the game. This is very annoying.