@Loh Jane,
There is an oral contract, at least in the US. Don't know about Singapore. Oral contracts can be difficult to enforce, of course, but they are contracts.
This difficulty led Samuel Goldwyn to remark that "A verbal contract isn't worth the paper it's written on.
Sam was under the impression that 'verbal' was the same as 'oral'. My understanding is that a verbal contract is either written or oral.