NEC ECC: Option A contract can a client request for cost omissions against a CE

My contract is NEC Option A, the client has requested cost savings on a submitted CE. Are omissions due on this type of contract

Yes if there is an instruction to change the Scope this will be a compensation event. This could be for additional works and lead to an increase to the Prices, or could be for an omission of works which would lead to a saving. You would produce a quote to show the amount it would have cost you to have done those works (less any cost you can prove has already been spent) and also include fee in that saving as well (i am afraid). Any compensation event is assessed as Defined Cost plus fee - it is just in this case it will be a negative number.