I've got a 5 1/2 year old too and we're chugging our way through OPG. We don't read the stories for comprehension in OPG. It's just phonics practice. I like it because there are no pictures and no guessing, purely sounding out words. I have her re-read sentences for fluency, but never comprehension.
We just got over the hump you are describing. We've been on and off or a year, but now we are officially doing "school" because this is Kindergarten year. What has worked is doing a short 10 min. lesson 3 times a day. 10 min out of OPG - pure phonics instruction, 10 min. out of a bob book.. something more fun to read for comprehension, 10 min. reviewing a lesson, or another easy book, whatever. The key was three time a day, short lessons, every day... being consistent. Last week it just clicked!
I've also learned to be a little more patient with dd. I expected her to catch on more quickly than she has, but finally figured out I need to let her go at her own pace and not rush through OPG, but rather take our time, and be diligent in teaching. Maybe in addition to shortening the lessons you could back up 10 lessons or so, work on something easier and practice fluency...??? I'm no expert, but that has also helped with dd's reading as well.
Good luck!