OptionSet is a DataType and OptionSetType is a VariableType.

The intention is that you either use the DataType to represent a BitMask or the VariableType.
As a guideline you would rather use the DataType when the BitMask is fixed and used for several Variables whereas the VariableType when the BitMask is specific for that Varaible.
I think the main reason for adding the DataType (the VariableType has existed before) was that it allows the Client to write only some bits of the BitMask. With the VariableType this is not possible. So if you just want to change one Bit using the VariableType, you would need to read the Variable fist, change the one bit to what you like, hope that the BitMask has not changed and write the full BitMask.

So in a nutshell: Both are independent concepts you can use in OPC UA to represent BitMasks. We added the DataType later because it provides a required feature the VariableType does not have.

Enumeration DataTypes and MultiStateDiscreteType VariableType are exactly the same thing.

CMPWG Oct-22-2015:

Thanks for the clarification; this helps.

We request that some of this text be added to Part 5 to avoid confusion later.

Add some text to Annex A and add reference to OptionSetVariableType, and MultisetDiscreteType

In Part 5 7.17 OptionSetType an example explaining why the OptionSet DataType can be used as the DataType of an OptionSetType was added.

Agreed to changes edited in telecon.