Or if you prefer: Windmills? In Oakland? MooOOOooo! I'd noticed this windmill years ago, but saw it again when I was in the area shooting the Steele's sign a while back. According to this post, it's the last remaining vestige of Idora Park.
Idora Park was an amusement park, with rides, a baseball field, a zoo, and an opera house. It was built by the Realty Syndicate that also owned and operated what later became known as the Key System transit company, and lasted from 1903 to 1929. Oakland's own Jack London took his daughters there, riding the rollercoaster repeatedly until one was no longer afraid of it, because as he said, "No child of mine is ever going to be afraid - of anything!".
Check out this great collection of Idora Park postcards at AlamedaInfo.com and read more about Idora Park at Wikipedia.