I remembered reading somewhere that ovulation predictor strips could be used as a pregnancy test, as they react to HCG (human chorionic gonadatropin), as well as LH (luteinising hormone). There's some great information on justmommies.com, which i've had a read through today. The two hormones are apparently very similar in their structure, but HCG has more amino acids and the HPTs are designed to detect those acids.
LH is always present in the body, but just before ovulation there is a surge in the amount, and this is what triggers the ovaries to release an egg. OPKs are designed to detect this larger amount of LH. A HPT on the other hand will only react to HCG, which in turn is only present in the body when pregnant, so they are more sensitive and react to low levels of the hormone.
So an OPK could detect a pregnancy, but it's likely that the HPT will detect it before the OPK would.
This morning i took an OPK with FMU (first morning urine) at 9DPO. Sadly, it didn't even show a line. I've had no symptoms at all barring feeling nauseous yesterday afternoon, but that was following a meeting in a room without air-conditioning on a hot day (33C), so more than likely due to that rather than a possible pregnancy.
Reading the information above though, i think that it would be pretty unlikely for my body to have enough HCG already, at 9 days past ovulation, to be detectable on anything but an early pregnancy test, and i'm using internet cheapie OPKs. So i may try another one on Sunday, at 12DPO. I'm still undecided about using up one of my remaining two pregnancy tests, but i'll blog if i do.