I usually o cd 18 based on bbt. this month i had small temp increase on day 15, and opk peak fertility next 2 days which is more reliable?

Fertility test: There are more things that can disrupt your basal body temp. such as infections and activity so it is less reliable but either way it sounds like you ovulated this cycle.

Ovulation prediction: Good morning, Margaret! BBT testing is good at telling you ovulation has occurred if there is that approx 1 degree temp shift. It's poor at letting you know precisely when ovulation occurred, though (i.e. some time with about 4 days preceding the temp rise). OPKs are better at helping time intercourse, with best chances within 2 days of the LH peak. Doesn't sound like d15 rise was ovul. Good luck!