I added one of the dryers shown below. Not sure if it's "Needed" given that I'm pumping indoors, but given the total investment in my rifle, I thought it a reasonable addition.
It is strongly recommended. The last thing you want is moisture getting in your tank or bottle. I use the Diablo Dry Air System (http://www.airgunsofarizona.com/pcp-accessories/diablo-dry-air-system/), same as Mousefart. You also need to manually bleed the moisture every 15 minutes as the Omega Air Charger does not have an auto-bleed. This is accomplished by backing out the black knob at the bottom of the small housing with the gauge on top for a few seconds. Just be sure you snug it back, or the compressor will not build up pressure.