Any contract, including an employment contract can be oral. However, there are certain types of contracts which MUST be in writing, as governed by the Statute of Frauds. These are as follows:
Contracts involving the sale or transfer of land;
Contracts to answer for the debt or duty of another;
Contracts that, by its terms, cannot be completed within one year. This means that the actual terms of the contract must make it impossible for performance of the contract to be completed within one year); and
Contracts for the sale of goods worth at least $500.
An oral modification of a contract is valid if there is new consideration on each side. Consideration means that both sides have to be giving each other something of value when modifying an oral contract.
To answer your first question, most oral contracts are enforceable. The problem with oral contracts is not usually their enforceability, but proving the terms of the agreement. Without a writing it is usually a he said she said dispute to prove what was agreed to at the time.
Most written contracts provide that the written contract cannot be modified unless in writing. However, even under those circumstances it is possible to argue that the agreement was changed or rescinded or cancelled, or that a new and different agreement was entered into. In other words, there is no guarantee that because there is a contract in writing that the relationship cannot be changed through words or conduct. However, the better the contract, the better chance of avoiding the arguments that it was modified by words or conduct later. Thus the need for a good attorney to nail down solid contractual language and process.
Good luck to you.
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[1. Is an oral contract enforceable in employment law?]
Yes, subject to exceptions (e.g., Statue of Frauds).
[2. Can a written employment contract me modified orally?]
It depends on whether the written agreement has a contractual provision that says all future amendments and modifications have to be in writing in order to be binding and enforceable.
Although a oral contract may potentially be enforceable, you will need to show and provide the intent and terms of the oral contract between the parties. Certain terms can be proven by evidence of performance or other action (e.g., performing some act or making some payment on a certain date), but it may be difficult to prove the oral contract's detailed terms and conditions without some writing(s).
I would recommend that you engage a labor & employment attorney with entertainment experience who can review all the facts relating to your situation and advise you accordingly.
The foregoing response is provided for general informational purposes only and is not a solicitation for business. Please retain an attorney if you need specific legal advice. No attorney-client relationship is established until both you and me agree to establish one, and neither transmission of information herein, nor the receipt of such information, constitutes an agreement to establish an attorney-client relationship.