The implications begin with the fact that Oh Dae-su is punished for a trivial act that he didn't even intend to cause harm. In this light, the man on the top floor, with his (electronic) omniscience and omnipotence, is a deity who oversees a universe in which punishment comes many sizes too large for the crime. Oldboy thus makes good on the promise inherent in the schematic narrative means of quest romance: Oh Dae-su's imprisonment and rebirth into pain, and his traumatic search for an explanation, present an emblematic view of life as a spiritual trial. (This potential is almost always ignored by action moviemakers.)
But when God is a pathological sadist, as the man in the penthouse is here, there can't be happy answers to spiritual questions. You also have to keep in mind that at the time of his kidnaping, years after high school, Oh Dae-su is an ill-behaved mid-level businessman out getting blitzed on his daughter's birthday. (He never gets to give her the strap-on angel's wings he'd bought her.) Oh Dae-su is thus fallen man (i.e., out of sync with the angels) who in some way deserves his torment. (The fact that the events in the movie derive from the two main characters' interaction at a Catholic high school justifies discussing the movie in Christian terms, though it isn't necessary. The movie also seeks to reflect recent Korean history, which I won't comment on here.) In Oldboy redemption for suffering takes the form of an attempt to bring the suffering back to the author of it, which naturally doesn't work. In Christian terms, in fact, not turning the other cheek indicates further indulgence in sin--how much more so when you're out for revenge against God for the basic conditions of existence. Of course, the evil deity of Oldboy can't be punished; he can kill himself, however, which makes him that much more remote.
The fact that I can come up with a coherent reading of Oldboy as a spiritual narrative doesn't make the director Chanwook Park a great romancer. His style does have a fascinating combination of impersonality and obsession; it's as if he had filmed this enflamed material expressionistically using a surveillance camera. At the same time, however, the movie approaches us as if we were both a primitive religious congregation and a benumbed action-picture audience, in either case a group in need of shocks. Park delivers his allegory --an icy rather than fiery vision of hell--with intensity, but his vision lacks variety, of incident and of tone, a drawback in heroic romance, which ordinarily uses fantasy and pageantry to beguile us into its visionary realm.