The ornamental design for an ottoman, as shown and described.
FIG. 1 is a front, left perspective view of an ottoman embodying our new design;
FIG. 2 is a left side elevational view thereof, the right side elevational view being a mirror image thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a rear elevational view thereof.
The broken line representations of wheels in FIGS. 2, 3 and 5 are for the purpose of illustration only and form no part of the claimed design. The bottom view forms no part of the claimed subject matter. In this regard, it is not visible in normal use.