We claim the ornamental design for a microwave oven, as shown and described.
FIG. 1 is a perspective view of a microwave oven showing our new design:
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left-side elevation view thereof;
FIG. 5 is a right-side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines in the drawing figures are included for the purpose of illustrating environment and form no part of the claimed design. The portions of the article shown in broken lines form no part of the claimed design.