Definitely best to have a holster. The attorney general opinion that defines OC as not brandishing mentions holsters.
What kind of gun do you have? Shouldn't be too hard to find a holster that would fit. Dunhams, Dicks, army/navy stores, they'll all probably have holsters that will fit.
Just the same, if you don't have a holster, it's not essential that you OC. Remember, we like to be armed, that's what we're about, but the picnic is about meeting people.