the one month that I used opks, I got m bfp, but with no thanks to them, I assumed they werent working (got them free from work as out of date) as every time I used them it would come up that I was ovulating! (wtf??) I used the very last one two days before af due and yep it came up pos, didnt think anything of it other than thank god I didnt pay for them, then the next day used a hpt and got my bfp, then it all made sense!!
so in my experience yes, they work as a hpt. not sureabout the accuracy though.
good luck.
xxx