Well, by the time you would get a positive on an OPK, you would have a blaring positive on an HPT since the OPKs are not as sensitive - but yes, you can use them.
http://www.peeonastick.com/opkhpt.html
The only thing is that you can have a line show up pretty much at all times on an OPK and I know a lot of women have a darker (but not positive) OPK right before AF - so it's kind of a crapshoot - I'd just stick to HPTs.