You have to copy the needed OpenCV DLL files(and maybe other DLLs which come with the compiler) along with the executable (in the same folder - or the system folder).
The easiest way is to copy opencv_world420.dll (try to link to the release version of the library, which doesn’t end with “d”). If you want to reduce the file size, rebuild OpenCV without opencv_world and copy only the needed DLL files (opencv_core420.dll, opencv_imgproc420.dll etc.)