It’s apparently an issue with OIIO using a function/command that isn’t in Win 7. Apparently compiling with mingw fixes that, as mingw includes that function/command. However, doing so ALSO breaks it for other builds.
At any rate, the dev said he’ll upload sometime this week a new libOpenImageIO.dll that should be compatible with Win 7. Crossing my fingers that that works.
Relevant Thread: To bug or not to bug ^^ - #14 by artao5