I went on an interview and the partner asked me if i've written a memorandum of law. I said yes, but I have not written any in my writting class. I tried to look it up but I came up with "legal opinion". Is a memorandum of law the same as a legal opinion? Which do firms use more often?

A memorandum of law is often used internally by a law firm. Law clerk are often asked to write these memos to assist the attorneys with their reseach. Legal opinions are often written by an attorney and addressed to their clients. Legal opinions are very costly because the user of the legal opinion can rely on the opinion in a court of law. So, there is a lot of malpractice exposure associated with a legall opinion.

Law firms, such as the one I work for ask all new associates to furnish a writing sample such as a memorandum of law prior to hire so that the hiring partners can determine whether the candidate has the necessary skills to assist the attorneys.

If you dont have a memo of law for a writing sample, then I would create one anyway. Ask your prof for sample issues and formats.