I claim the ornamental design for the octagon post cap, as shown and described.
The cap has an opening that receives a end (usually a top) of a post. The cap is associated with deck posts and the bottom of the cap is intended to be used with lighting elements.
FIG. 1 is an isometric view of an octagon post cap according to the present invention;
FIG. 2 is a top view of the octagon post cap;
FIG. 3 is a bottom view of the octagon post cap; and,
FIG. 4 is a side view of the octagon post cap (the sides have generally the same appearance).