I have been charting now for more than 20yrs. OPKs weren't even on the market when I started charting, lol. Not all women are designed for temping as it requires pretty consistant timing and daily practice. Temping only shows you AFTER ovulation has occured. It is a very useful tool in pinpointing your normal body cycles. The temps need to be taken at the same time every am upon waking b4 your feet even hit the floor.
Like one of the previous posters said, you may get a pos opk and not o until some time later. I have even had pos opks that turned out to be random surgesm then got another pos opk later and temps confirmed it was the real one. Some women will see as much as a full degree difference pre and post o. Others may only see .20 shift which can be hard to distinguish.
Here is my word of advice about charting and using OPKs. Last month I got a positive OPK on day 12 (evening) and DIDN'T ovulate until the 15th! If I was charting my temps I may have stopped BDing too early! Don't assume the 24-48 hour thing!
For some people monitoring your BBT works. Mine is all over the place. I was told by my dr that the only thing BBT is good for is to see when you ovulate - that there should be a spike. This month, I didn't have a temp spike, had a positive OPK and an ultrasound said that I had ovulated... The nurse said to use an OPK b/c we can't rely on my temps...
OPKs detect LH surge. However, it's difficult to say if you ovulate within 24 hours or 48 hours after the surge is detected. Your BBT can tell you when ovulation takes place although I think most people use BBT retrospectively... Also, during tww most people watch their temps since they give an indication of the upcoming AF or if your temps keep being high or even increase it may mean pregnancy.