1: ὀπίσω
(Strong's #3694 — Adverb — opiso — op-is'-o )

connected with hepomai, "to follow," is used adverbially, of place, with the meaning "back," "backward," in the phrase eis ta opiso, lit., "unto the things behind," in Mark 13:16; Luke 9:62; 17:31; John 6:66; 18:6; 20:14 . Cp. Philippians 3:13 , "the things which are behind." See BEHIND.

2: ὄπισθεν
(Strong's #3693 — Adjective — opisthen — op'-is-then )

of place, "behind, after," is rendered "backside" in Revelation 5:1 , AV (RV, "back"). See BEHIND.