For any given regular polygon:
Apothem is a line segment from the center to the midpoint of one of its sides. Equivalenty, it is the line drawn from the center of the polygon that is perpendicular to one of its sides. Thus in our case, the apothem is OP. This is because OP is perpendicular to AB.
Radius is the distance from the center to any vertex. It is always the same for any vertex. The radius is the radius of the polygon's circumference. It is also know as circumradius. In our case the radius is OB or OA.
Perimeter of the polygon is the total distance around the polygon. In this case it will be given by:
p=AB+BC+CD+DE+EF+FG+GH+HA