"Omissible" means that something can be skipped or left out, which suggests the opposite should refer to something that must be done or included. In this case, the antonyms might include essential, necessary, required, or compulsory. Alternatively, antonyms could focus on the opposite of being overlooked or forgotten, such as acknowledged or recognized. Other possibilities might include included, incorporated, or counted. Whatever antonyms are selected, they should reflect the idea that the original word refers to something that can be omitted or ignored, a characteristic opposite of a term that denotes it as necessary, mandatory, or worthy of attention.