Water is great as ballast, especially since you have no weight to lug beyond the water's edge.
If you don't mind carrying weight though (or need to maximize space in the front cockpit) you can fill your drivewell plug with lead shot -- I got this idea from Stringy who also has an Oasis. I use a second lead filled cylinder (melted wheel weights) stuffed in the bow through the front hatch. Together they weigh 55 lb. Works great for me! When placed this way, the weight is kept very low and adds an excellent righting moment when sailing.
