Is None less evil than null?

The problem with null is that you have the possibility to use it almost everywhere, i.e. introduce invalid states where this is neither intended nor makes sense.

Having an 'a option is always an explicit thing. You state that an operation can either produce Some meaningful value or None, which the compiler can enforce to be checked and processed correctly.

By discouraging null in favor of an 'a option-type, you basically have the guarantee that any value in your program is somehow meaningful. If some code is designed to work with these values, you cannot simply pass invalid ones, and if there is a function of option-type, you will have to cover all possibilities.