Ovulation usually happens about 48 hours after the beginning of the LH surge, but has been noted by doctors to occasionally occur up to 72 hours after the beginning of the surge in women with normal fertility. Temps can take up to three days to rise post-O in some women, so it is still borderline normal to have a positive OPK and not see a temp rise until almost a week later.
If LH surges but O doesn't occur after the surge, the body will often try to ovulate again about a week later.