From the impf/pf pair опираться/опереться (=to lean on), опереться seems to be the only verb that exhibits the following strange conjugations, though apparently belonging to the -e- conjugation group:
Future: обопру́сь / обопрёшься / обопру́тся (the prefix об- suddenly appears, with an apparent disappearance of the first mobile e in all forms?)
Past: опёрся / оперла́сь / опёрлись (no final л in the first person singular?)
Imperative: обопри́сь / обопрёмся / обопри́тесь (again, with the prefix об-?).
Could someone share the etymological or historical reason for such strange conjugations, in particular the appearance in the future and imperfective of the prefix об- and its meaning?