Cherry is correct. both can function as an adjective but only “optimum” can be used as a noun. In fact, there are some people who believe that “optimum” should only be used as a noun and it’s only because people have repeatedly used it as an adjective (incorrectly) over a long period of time that it has become acceptable as an adjective. So if you want to be safe, it would be best to only use “optimum” as a noun. But I’m sure no one would correct you if you used it as an adjective.

They have both the same meaning and are interchangeable. The best level something could achieve is the optimal level or the optimum level. - For optimal results ( the best possible results) - optimum ecological conditions.