OneLoader is the de-facto standard ModLoader to use for modding OMORI, and installing it is a required step in order to begin using or creating mods.
NOTE: If you have previously used GOMORI, It is recommended to first perform a full re-installation of the game prior to continuing. Alternatively, you can use the One Loader Installer GOMORI mod, though it's reliability is uncertain in some scenarios.
Video showcasing the procedure
After you install OneLoader, install them as follows: