If you are trying to run a "standalone" OptiCraft .exe file rather than installing it as a mod via a launcher:
While the fixes above will make Opticraft (or standard Minecraft 1.12.2) run at a stable 60 FPS on Windows 7, the security risks are now too high. Multiplayer servers require modern TLS 1.3 encryption, which Windows 7 cannot natively support without third-party patches. opticraft minecraft windows 7 fix
: If the modern launcher fails, try the older "Windows 7/8" launcher available on the Minecraft Download Page. If you are trying to run a "standalone" OptiCraft