Casual early dinner in Oceanside
I use to be a member at arrowood, decent course, if you are coming from overcrowded la/oc courses, it is a great course. I would highly recommend Marietas mexican on Vandegrift for dinner/drinks. It is on the way to arrowood. It is really good mexican, I usually get the enchiladas suizas and a carne asada quesadilla. Their chips and salsa are great too. This place is a bit out of the way for most people but since you are going out that way I highly recommend it. here is a link.
You didn't mention what part of Oceanside you'll be in, but if you're playing Oceanside municipal or Arrowood then let me recommend Capozzoli's. It's a family-run Italian restaurant on the SE corner of Douglas and Mission and it meets your criteria for an early, casual dinner.
Another solid nearby option is Ocean Thai Cuisine.