A circular saw (battery or plug-in), set the thickness of the plywood, will probably give you the straightest and cleanest cut. Assuming you want a straight cut that is. Setting like this will let you cut over the top of the framing members without damaging them or the saw. Use it to cut as close to the corners of the opening as you can, and then use a jig saw, oscillating saw, or even a saws-all to finish the corners. I would use an oscillating saw if the corner is on top of the frame.