First, be sure to take a picture of the graffiti, and note down the closest street intersection. Then immediately report it:
Please report this to CalTrans. Use the "Graffiti" option in the drop down selector for "Select Situation Type". Note the ticket number.
Report this using Oakland's 311 service. You can simply dial "311", or you can report online using this form. Note the ticket number.
If it's on private property, report it to 311, and then also report it to the property owner.
It's very important that you share it with your city council member. Send them the ticket number with the picture that you just took. Use this interactive map to locate your representative and send them a message.