Congratulations on your new Oasis! To start out, a plug-in cart is a great accessory -- lets you roll the boat instead of carrying it. The "standard" cart will easily handle the Oasis in all but soft sand. Personally I really like Hobie's i-comfort seat pad for greater seating comfort but a lot of folks find the standard seat bottom quite adequate. Other than that, the boat comes pretty complete IMO.
The roller bearing Drive is new for 2015 -- makes it easier to pedal. I don't think it's anything you need to get at this time but maybe something to look into down the line.
There are plenty of tips on the forum discussing safety equipment, maintenance, rigging, loading and transportation, etc.