When starting a C++ program, The following error pops up: A required .DLL file, OPENGL.DLL, was not found.

assuming you built the program
you could link opengl32.lib (for windows)
not opengl.lib

anyway without that change either put opengl.dll (or opengl32.dll) in the folder your app is in

or better yet in the c:\windows\system folder

needless to say if you dont have this you can get it from

for opengl32.dll
for opengl.dll