The Fourth Sunday Race (aka Fourth Sunday Run) is a race series at Lake Merritt organized by the Lake Merritt Joggers and Striders running club the fourth Sunday of every month except February (Couple's Relay) and March (Oakland Running Festival).
There is a 5k race, 10k race, and 15k race each month and a kids runs. The kids races start at 8:40am and the rest of the races start at 9am.
Registration fees for the races are the best bargain around: from $3 for a member who preregisters to $7 for a non-members who registers at the race.
The official page includes information on dates and registration.