I think that Ohba is a girl. Or a girly man. My reasoning behind this is all of the subtle hints of Yaoi and that in it (sorry dudes)
Examples *ahem*
1- Light and L were joined with Handcuffs. HANDCUFFS!!
2- In the last chapter, Ide says that Matsuda was 'Fond' of Light, and Matsuda gives a sad expression
3- Matt calls Misa Amane pretty, and Mello's first reaction is "Are you Serious?" XD
There are other hints as well, like Mello apologizes to Matty. I know that I'm being fairly sexist here, but that seems like the kind of thing that a girl would think of adding *shrug*
As L would say in a situation like this "Whatever choice you choose, Male or Female, there is a 50% chance that you are correct and a 50% that you are incorrect. Whichever you decide, there is not enough percentage to be absolutely sure of your decission."