opks can be taken when you want. they say not to take them in the morning because a pos may not show up..but if it does for you its fine. It does for me. some brands say not to use with FMU others say any time of day is fine.
LH is synthesized early in the day, and is not metabolized into your urine until later. So, as a rule, you are more likely to catch your surge later in the day. ("They" say you should test sometime around 2-4 p.m., but I've had fine luck at 10 p.m.-midnight.) Using FMU may not be a problem for you, but if you do not detect a surge during the cycle you use FMU, try testing later during the next cycle.