Hi Raideress
I used an OPK as a HPT at 6 and 8 dpo, both times coming up negative. At 12dpo I tested again using a HPT and it was positive.
I have also heard of people getting positive OPK's only to then receive negative results on HPTs a couple of days later.
For some people it may work, but as you cant really be sure that the results are accurate I would not recommend using them as a HPT.
Hope this helps
Good luck with whatever u decide to do...