An oral contract is, in most circumstances, just as enforceable as a written contract. Of course, the law always prefers written agreements because it is easier to define each party's rights and obligations by looking at a document. So if you have an oral agreement with another party, it is almost always better to put it in writing. This avoids confusion later. But if you do put an agreement in writing, it's important to know about the "parol evidence rule" and how that might affect what is in the contract, and what is not.