I used the ones from the dollar store before I got my clear blue fertility monitor and they worked just find for me.
i know i bought a target and a cvs pregnancy test and it was the same exact design. so the opk might be the same too.
in meaning that, those OPKS (any brand really, I tried so many) never showed that I had ov. I even used the saliva strips, too.
So, for me, never showing a + was correct.
I used those and they worked great.