I claim the ornamental design for a microwave oven, as shown and described.
FIG. 1 is a top perspective view of a microwave oven of the present invention;
FIG. 2 is a front view thereof; and,
FIG. 3 is a side view thereof, the opposite side being a mirror image.
The ornamental design which is claimed is shown in solid lines in the drawings. Any broken lines in the drawings are for illustrative purposes only and form no part of the claimed design.