Quoting Vindictive:" I'm surprised at how many people are saying yes. An OPK (typically) doesn't gradually get darker like a HPT. And a lot of women surge so quickly, that if you don't test twice a day, you'd miss it."
I've used OPKs for many years now. Rarely do my OPKs get progressively darker as ovulation approaches. This past cycle I hardly had a second line on my OPK the day before ovulation (I tested 3 times a day)