This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
If the parties purposely omitted a part of their oral agreement from their written contract, no mistake exists except possibly in their belief that they can prove the oral contract and enforce it as well as the written one. In cases of this sort equity does not grant reformation.1 Elementary as this proposition may seem in view of the so-called parol evidence rule,2 there is some authority for allowing an oral term of a contract agreed upon before the rest of the contract was reduced to writing and executed, to be added thereto by reformation.3
A written contract cannot be reformed by adding a provision agreed upon by the parties orally after the written contract was made.4