babykk,
from what i read, bbt is most accurate to tell whether O or not.
opk can only tell you that body might / will be ovulating, but not confirm de.
only temp shift will tell.
though usually by then, too late to BD.
thus best to measure for few cycles to roughly know when body will ovulate.
like for me, i usually ovulated between cd18-cd21.
regardless how early my EWCM appears, somehow, the bbt will only shift around those dates.
when gg to ovulate, temp will actually dip.
then next day, raise at least a 0.3-0.5 shift ...
of course, some do shift gradually...