I'm told that yes, "octopus" is a third declension word, but that does not make its plural "octopera". The proper plural is, in fact, "octopodes". The person claiming "octopera" is probably confused because "opus", another third declension word, has as its plural "opera", but that's because the real root of "opus" is "oper". Also, since "octopus" is masculine it can't possibly have a nominative plural ending in "a".