FIG. 1 is a front view of an operation indicator of a heater showing my new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof; and,
FIG. 6 is a perspective view thereof.
The broken lines in FIGS. 1 through 6 are shown for illustrative purposes only and form no part of the claimed design.