I have been playing with the CoB with a BSB Death Hag on it for a while now in a horde of witches.

However we keep having some unclarity on the exact rules:
- How does shooting on the CoB work? Can you target the Hag or do you have to randomize and so yes how?
- In close combat I assume you can target the Hag directly on its own stats? Doe the CoB provide any stats boosts in that case (like As)?

Probably posted somewhere already but I couldn't find

Thanks in advance!

You can find the rules in the BRB pg 105, "chariot mount" and onwards.
In short:
- Shooting is distributed as shooting attacks on the unit. You can not target the COB individually.
- Templates hit both COB and Hag
- In close combat, the Hag can be targeted specifically and she gets a 6+ Armour Save from being mounted (on a chariot).

Thanks for the quick reply.
However in that part of the rulebook is stated that a chariot can not join another unit.

1. It sounds like me that because of this you should be able to target the cauldron and randomise
2. Next to the hag it has two other riders. Do you need to randomise on them as well?

Yeah it's a little confusing. The riders count as crew of the chariot, and therefor can not be targetted specifically. The randomizing rule comes from the rules for characters joining a unit, which a COB can do as its rules specify and the specific rule from the AB trumps the general rule from the big rulebook.