The harbor in Oceanside is the last actual beach located in San Diego County. The harbor itself is located in North Oceanside and is just North of the pier. The roads that lead around the harbor can be somewhat tricky so make sure to follow the signs to where you wish to go. As you arrive at the Oceanside Harbor, you will see an abundance of parking spots and lots to park in. However, most of these lots require a fee to park or are toll parking spots. The fee is $5 - $10 depending on how long you will be there. If you turn right as you see the beach and head to the boat ramps and more additional parking. These spots are reserved for people using the ramps and such so please park either in a pay lot or park further away from the beach and walk in.
The beach access is pretty straight forward at the Oceanside Harbor, walk about 200 yards and you are in the water! It doesnt get much better than that in my book so enjoy the soft sand and fun waves.
There are a number of conveniences and useful things located at the Oceanside harbor. First off, there are many public restrooms located near the beach and parking lots. These restrooms are maintained by the city and are usually decent in cleanliness. On busier weekends, make sure to note that they probably havent been cleaned in too long so wear some sandals or shoes in there.