Both are good indicators of ovulation, but I rely on BBT charting. OPK gives you an idea about when O should take place. Bbt helps you with when it takes place. But you need to spend few months of practicing it to get a hang of it and gain understanding of pattern to find out your most fertile period.
OPK usually works on principle of LH surge in luteinizing hormone (LH). This surge is what causes your ovary to release an egg. However, for some women, LH levels are still relatively high even after ovulation. And, depending on the brand of OPK you used, it might be able to detect relatively low levels of LH. Thats why OPK could be misguiding.
Alternatively you can get negative OPK if you just missed the LH surge or LH is low that OPK has been unable to detect it. But if your temp spikes, then you can consider that you did ovulate.
Always keep in mind, an OPK can predict ovulation, but doesn't confirm that it definitely happened. Only your temperatures (or an ultrasound, or a pregnancy) can do that. Try charting BBT at the same time of the day, preferably, first thing in the morning.
