I claim the ornamental design for the office chair, as shown and described.
FIG. 1 is a perspective view of an office chair showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines representing stitching shown in the cushion in the FIGS. 1-2, 4-7 are part of the claimed design; the broken lines shown in the chassis and wheels in the FIG. 7 depict portions of the office chair that form no part of the claimed design.