The ornamental design for a combinationoven, as shown and described.
FIG. 1 is a front perspective view of a combination oven showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a left side view thereof; and,
FIG. 5 is a top view thereof.
The broken lines depicting the vertical seam and the elongated racetrack-shaped region within the shaded area form no part of the claimed design. The broken lines depicting the remainder of the combination oven form no part of the claimed design.