OPK not lining up with BBT chart

[upwardonward]

I am new to temping. I've been doing it for like two weeks. I'm having a hard time taking my temp at the same time every day, because I just don't wake up at the same time every day. So, I have set myself an alarm before I would ever normally wake up so I can get a temp at the same time every day. Is that acceptable?
But, back to what I titled this thread. According to my chart I ovulated on the 27th, but I got a positive opk on the 28th? What's that about?