Dreamin, on OPK is an ovulation predictor kit....it basically measures the LH in your urine which peaks about 24-36hrs before OV....I guess its just another measure to make sure they dont miss ov. They dont work for some people but have always worked for me.
I had long cycles and my natural FET was always just the same length (about 35days).
I had 7 transfers all up....5 were FET's (all natural but did ask for progesterone support on the last one just to try something else) and I got 4 BFP's 2 chemicals, 1 MC and this one where I am now 33wks. So for me I had alot of success with FET's (all mine were either 2 or 3day embies).