I hadn't seen too much by way of comparison, but a search I just did shows that a few bloggers have tested this and found slight differences.
Since I found a lone bottle of Orly Fowl Play during my most recent trip to Ulta, I snatched it up since I knew I had OPI Merry Midnight at home. I don't normally do nail polish comparisons, but thought this one would be fun for a change.
Please keep in mind these are just my opinions on how the two polishes compare. I'm by no means an expert, this is just to maybe help those out who are looking for either polish or are trying to decide if they need both. Please click the pictures to enlarge them, if needed.
It is my personal opinion that OPI Merry Midnight has a lighter base than Orly Fowl Play. The shimmers seem the same, but I believe the Orly to be more dense. I actually prefer the Orly over the OPI. For reference, I did 3 coats of each polish.
The original price of OPI Merry Midnight was $8.50, but it's going high on eBay. Seriously, in the completed listings one sold for $37.00. Orly Fowl Play is not a complete spot on dupe, but it might help your lemming a bit. Especially since it's only $7.00.
Orly Fowl Play is now available at Ulta and Sally Beauty Supply.