I'm assuming you want to make an angled oval instead of a smooth curved oval. I have a fairly simple method. The basic principle is to start with a rectangle drawn on paper for the outside dimensions you want.
But, you only have to draw half the length/height/width, whichever way it's laid out. IOW, lets assume the overall size will be 18" wide by 36" high, and the bottom and the top will be angled to create an oval look.
Just draw out from the centerline at 18", and that half will be the same as the other half. So, you have the outside line drawn, and now draw a line parallel to that line for the inside line, which represents the width of the frame borders.
You could start with an easy method of using 45 degrees for the angles. From one corner of the top half of the rectangle measure down in even increments starting lets say at 4", and put marks at every inch. Do that in both directions from the corner. That will give you a 45 angle. for the corners. The further down you mark off, the longer the angled piece will be but the angle will stay at 45 degrees. You end up with basically 8 pieces to the frame, like this
You can stretch out the oval by making the two parallel sides longer, or change the shape of the ends by offset marks for the angles, i.e., lets say 8" down and 6" across. That will change the shape and the angles will no longer be 45 degrees.
Once you have the half drawn out, transfer to a substrate to make a template for the frame. You can rough cut the final material, and then trim close with a flush trim bit.
Or, just cut the 8 pieces of wood with the angles you drew. You can fit them right to the drawing to check the fit. From the drawing, you can draw in the rabbet for the mirror size, and use that to get your mirror cut for a perfect fit.