The summer season is really the most pleasant and happening time for Oceanside.
The beach is generally packed and Oceanside is one of the few beaches that allows "cruising" along the street next to the beach. I am not a fan of either the packed beach or cruising, but I know some folks are and generally the teens love it. July 4th brings a mass of people, bands, volleyball, and lots of alcohol consumption, Fun time of year.
Keep in mind that this area of Ca. has an eclectic bunch of people, a large military base, and tattoos and motorcycles are common. Although I am told that this area used to be crime ridden and rough that is not an issue for the most part. But, it may not be suitable for young children or the faint of heart.
Oceanside is a very popular surf site and since my surfing ability is limited to the few lessons I have taken I must rely on other sources to say whether it is good surfing. I assume it must be good because they arrive before 6am even in the winter, 50 or so bobbing heads waiting for a wave, and surely freezing their tails off.
this is a great walking beach - plenty of sand dollars and easy breaking waves. Not a lot of rocks either.
enjoy the sunset - but dont drink on the beach! Fires only in designated areas or on private property....