If you are going to be cutting a lot of circles and need high precision, get the MFS. Search the forum for a video - the MFS is not intuitively obvious, but its a sweet tool when you get to know it.
For occasional circles, just get some longer screws for the OF1400 baseplate and attach it to the end of a piece of 1/4 ply or mdf thats a foot or so longer than your circle's radius. Plunge the bit thru the ply to mark where the bit will cut. If you want a 24 inch wheel, measure 24 inches from the inside cutting edge of the bit and mark that spot for your pivot point. Drive a nail thru the pivot point mark on the ply into the workpiece where the center of the circle should be. If you want a 24 inch hole measure from the outside of the bit.