The way I see it (referring primarily to the novel), is that the relation between Oskar and Eli indeed is love. However, their love is not romantic, i.e. it as not sexual component. It is the love one you may find between siblings or very close friends.
And I would be 100% on bored with that line of thinking if it was not for two things.
1. The kisses. Now I guess since we are talking about the book now this point is somewhat mute because you can argue the book kisses were mearly for sharing memories, but I think there is a possibility to argue it was more than that.
2. To me it makes perfect sense that two 12 year old kids (more or less) could have a romantic relationship without it being sexual in any way. I'm my mind you can have romance (or perhaps a better way to put it would be to say a 12 year olds version of romance) without the sexuality.
All the hints of a relationship, at least on Oskar's side are there. He buys Eli gifts, He wants to impress her, he asks her to go steady. And after he finds out that Eli is genderless, he then asks a teacher about being gay. Clearly Oskar is still wrestling with these feelings for Eli and trying to understand them.
Are those feelings the love of a best friend and sibling? Or is it something more?
Like I said, I could see it going either way. Saddly only an older Oskar could tell us for sure.