I've only read about using opks as hpts on the internet, and I am careful to take everything on the internet as potentially false, but the explanations make sense to me.
As I understand it, there are two componants of LH (which is what opks detect). I am going to call those two componants A1 and A2. I have no idea what they are actually called, but I know it has something to do with Alpha, which is why I picked A.
Also, as I understand it, HCG is made up of 4 componants. Again, I'm making up names: B1 and B2, but also A1 and A2 (the same A1 and A2 as is in LH). (the B letters have something to do with Beta, as in when people say "I got my Betas back and it is 307," or whatever.)
So, if an opk is designed to detect A1 and A2, in order to show "positive," it makes sense that it would also detect hcg, because hcg contains A1 and A2, plus some other stuff.
Again, I got this info from the internet, so who knows what it's worth. I'm just inclined to believe the stufff that comes with an explanation, more than I am inclined to believe the stuff that just states a conclusion. That explanation makes sense to me. On the other hand, I didn't believe I was pregnant until I got a positive hpt (actually, 2!). I wouldn't recommend using an opk as an hpt without more information because to me it's just not reliable. That said, it's fun, during the 2ww to have something to do, and I swear using that opk and pondering that for a few days kept me from using a hpt too early.