Oakland, California enjoys mild temperatures throughout the year, thanks to the Pacific Ocean influences nearby. While the city is located across the bay east of San Francisco and does miss some of that city's fog, its weather is not too far off from San Francisco in similarities.
The summertime high is a completely enjoyable 75 degrees average in the hottest month, September. If that's all the hotter it gets, you can fill in the blanks as to how you should dress when traveling to Oakland. Take a jacket all year long, and take a heavier jacket in the winter months. The moist bay air can chill your fingers on a drizzly night when the wind blows through the bay.
Rainfall is moderately light throughout the year, however. The most rain you'll likely see and experience will be in the month of January, and you probably will see some rain in February, as well.
Great months for likelihood of no rain at all are May, June, July, August and September.