It is not recommended to have an agreement entirely based on an oral agreement for various reasons. Although some oral agreements are enforceable under law, they are often difficult to prove and are easily debated in court.
Some agreements MUST be in writing according to law. The following types of agreements need to be in writing:
Real estate sales
Agreements to pay someone else's debts
Agreements that take longer than one year to complete
Real estate leases for longer than one year
Agreements for over a certain amount of money (typically over $500)