Typically speaking you dont have to meet any requirements when you are OCing, however it is suggested that you buy an outside the waistband designed holster (as opposed to the inside the waistband and then OC also known as virginia tuck) to avoid any gray areas. Also it is recommended that you have a good retention device on the holster. Holsters such as the Blackhawk Serpa, iTac, and Safariland retention holster to name a few. Those holsters by design prevent the firearm from being drawn by a mechanical means unless the retention device is moved buy your finger or thumb during the drawing motion allowing the pistol to come free. These holsters are designed to prevent the firearm from easily coming out of the holster and are suggested for use for OC to prevent the chance of a gun grab. There are also different levels of retention, if you search for retention holster you will come across the different types out there, also youtube videos are a great resource.
I OC a Glock 19 in an iTac retention holster and I absolutely love it. I've owned 4 of them thus far and I wouldnt hesitate to get another one.
