I actually bought two O-Ring kits; the standard one and the "all oranges" one. Can't have too many O-Rings. And I always keep my orange O-Ring on (the bottom notch of the core). The insulating sleeve comes off way too easily (for me) otherwise. I anticipated losing the O-Ring (which for some reason I haven't), hence my order of extras.
btw, I've been using the extended black delrin mouthpiece with UFO (aka "Tru-Flow UFO Mouthpiece"), and it's become my favorite mouthpiece to use directly into the Evo core. A minuscule reduction in airflow compared to the plain extended mouthpiece, and a minuscule less room for herb in the core, but NO MORE BITS IN THE MOUTH!!!
It also self tamps the load down for me, saving a step. (Fill the core loosely, put the Tru-Flow UFO in, and its flat bottom tamps down the load the perfect amount). My screen experiments were less than fruitful, so this is really a blessing.
Thanks for the tip, Tim!