We claim the ornamental design for a microwave oven, as shown and described.
FIG. 1 is a front 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 view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is an enlarged view of the encircled portion in FIG. 7.
The broken lines immediately adjacent the shaded areas represent the boundaries of the claimed design, while all other broken lines are directed to environment; the broken lines form no part of the claimed design. The dot-dot-dash broken lines encircling portions of the claimed design in FIG. 7 that are illustrated in an enlargement in FIG. 8 form no part of the claimed design.