Slice both ends of the prickly pear off. Make one long vertical slice down the body of the prickly pear. Peel back the skin, by pushing it back with a knife or your hand. The skin should come right off, and you should be left with just the prickly pear.
Place peeled prickly pears in blender with 1 cup of water. Blend at the lowest speed for 1 minute.
Strain and discard the seeds and pulp.
Place prickly pear juice back in the blender, with orange juice, and agave syrup. Blend until smooth.
Serve over ice.