your question is a little confusing because of the nature of FPGAs and the nature of software.
openCV is a library for writing computer vision applications, while FPGAs are hardware design utilities.
if you want to “use openCV on an FPGA,” then your FPGA will need some sort of processor IP core that conforms to an instruction set architecture. you will then need to compile openCV for that particular platform to generate the appropriate machine language instructions.
in a nutshell, FPGAs are for implementing hardware designs, not for running software.
you can accelerate certain applications by developing dedicated hardware to perform certain tasks, but in this case, that would involve going inside openCV and implementing some of the algorithms in hardware, then interfacing the FPGA with your PC and writing drivers so that your higher level software can take advantage of the hardware accelerator that you’ve just implemented.
i really don’t want to be discouraging, but it seems like you’re mixing two concepts without fully understanding each.
try to find which part of the problem you would use the FPGA to accelerate.