how accurate is an OPK

Hi everyone! Af isn't due for 8 days but I think i ovulated super early as when I have been pregnant in the past the scan has always suggested I ovulate not long after my period! I know it's way to early to test but if I was to take an opk in the morning would this detect the hormone? I didn't want to test until Tuesday but I'm super impatient! Thanks in advance cxxx