Confused...opk wth?

Samantha • •Living in wondrous chaos💞 •Mama to Isaac (8) && Vayda (2)
Yesterday I got a positive OPK which for me was CD15, I tested again today (12 hours later) and it was negative. So is today O Day then? Or do I keep testing? I know a bit about Ovulation but when things like this happen I get confused... HELP!