As Cotty and Box have pointed out, you can use the scale tool to squash (or stretch) a circle. I created a 3x5 ellipse and compared it to a scaled circle (both using 96 edges). I’m pretty sure that they are one and the same mathematically.
ovals.skp (21.8 KB)
As Gully pointed out, slicing a plane through the cylinder will give you the result “in place” without creating the ellipse separately:
