The ending of Let the Right One In has some serious Fridge Horror for some viewers. Many people see the end of the movie as sweet, since the child protagonist ends up together with the child-vampire friend (and possible romantic interest). The Fridge Horror comes from realizing that the creepy pedophile that had been caring for the vampire up to that point started out exactly the same way, and much of the plot was the vampire manipulating events to get rid of a servant who could no longer handle his duties — which means that in fifty years, the protagonist will be the old creepy pedophile, and the cycle continues...
Still horrific, but in a different way, is how this is handled in the source material. Her helper is actually just a guy she met when he was already old... while he was observing children on a jungle gym. This could be seen as a good thing though, as she may not treat the new friend as poorly.
The Americanized version, Let Me In, restructures some of the scenes and dialog to hint more strongly that this is in fact what's going on, though the question is still left open.
It's possible that Eli could "turn" Oskar, but that opens up another closet full of Fridge Horror...
There is also the possibility that Oskar doesn't help Eli out by murdering people. He doesn't kill the guy who woke Eli up, and also is quite shocked when he learns that Eli lives off the rings and money of the people her and her helpers murder. There is a pretty big difference between offing a complete stranger and hitting a bully on the side of the head.
An even more disturbing possibility...what if the immortal monster that "turned" Eli comes back, finds her with Oskar, and gets the idea to repeat history?
The teenage pimp may have been able to guess that Hakan was a pedophile because Hakan was looking around in a shifty manner that indicated he knew the place was a relatively well-known child prostitute pick up area, but why on earth did he constantly bug Hakan about the prospect of spending money on a child prostitute at a public library?