How to take BART to the Oracle Park
You have two options.
First, you could take BART to Powell St. Station and follow signage upstairs toward the Muni station. Once you exit the fare gates follow the "T" Muni signs to transfer to the Muni T Third trains at Union Square station. Screens located above you will show the next coming Muni Train. Hop on a Southbound T Line train heading toward Sunnydale. Get off the train at 4th and King streets and walk toward Oracle Park.
You could also take BART to Embarcadero Station and transfer to the Muni N Judah trains inside the station. Take the train toward Caltrain/Ballpark and get off at King & 2nd St, next to Oracle Park.

Check out the below rider guides, which explain how to get to some of the Bay's favorite venues.
Find other rider guides here: Chase Center | Levis Stadium | Oakland Arena | Oakland Coliseum | Oracle Park | Moscone Center