This topic recently came up in the "Do we have a tag for that" thread. oblivious and related tags have wiki pages, but naive does not. Given the similarity between these tags, I think it is worth delineating what the difference is.
I would argue the difference is that a naive character understands that something is happening but does not understand its implications, while an oblivious character does not understand that anything is happening at all. However, this definition also has overlap with confusion.
Proposed wiki description:
This tag refers to situations where at least one character misunderstands or is confused by something that is "adult" in nature (sex, violence, etc.) due to lack of knowledge. This can range from not picking up on context clues, to not understanding that sexual acts are sexual. This can sometimes overlap with oblivious, but while oblivious characters are completely unaware of a situation, naive characters are aware of a situation but unaware of its implications. Commonly portrayed with young characters.
See also
(It's really hard to find a good example image; I want a nonsexual one to distinguish it from naive_sex.)
Updated