Yeah Edit: Be sure to check this wiki page out on allowed and disallowed mods as well as other questions you may have about using mods on Empire Minecraft: https://empireminecraft.com/wiki/using-mods-on-the-empire/
I'm late, but be sure to check out the link Roslyn posted: But yes, OptiFine is allowed. A lot of people here use it, including my mom and very likely even some staff.