and Hcg are almost identical in make up. The last time I was pg, I got a bfp on an hpt at 11dpo. The OPK did not turn pos for at least 4 more days. You can read more on it at peeonastick.com. Elle has it all broke down in detail there :)
surge or the presence of hcg from pregnancy that can cause them to be positive for example PCOS and same fertility meds. I wouldn't rely on the cross reactivity of an OPK to give you an accurate pregnancy result. Go for the pregnancy test that is truly measuring hcg. Good Luck.