In this tutorial, learn to install Forge easily on your Minecraft server.
Installing Forge and its mods on its Minecraft server
Install forge version (example: 1.12.2-xx.xx.x.x.xxxx) on OMGSERV server by going to the "reinstall" menu, "Forge" then "version selection". If this one is not available, follow this tutorial.
Run the server for the first time.
Connect by FTP on the server with an FTP client such as FileZilla (tutorial) (or with WebFTP from the OMGSERV panel but this is not recommended because many bugs).
Delete the folder world (it's your world, deleting it will reset it, more information with this tutorial).
Move in parallel all your mods in the mods folder and your config folder if there is one.
Start the server and it will be accessible with all the mods.