I haven't done a test on this myself, but I have heard orbs do not count as shields, so you do not get the AC bonus shields normally give. I am not even sure you can bash with an orb. There is a reason why tanks don't use OoIV. OoIV is much better for casters since casters get very few high ac offhand items, and the mana/hp is awesome.
There also isn't any block mechanic when wearing a shield. The only class that gets "block" is Monks, and it isn't shield dependant, it is basically just another parry that further reduces the number of hits you take. The only benefits you get from a shield are the AC bonus (shield AC is not softcapped), and you can use bash.
I would just use an
https://wiki.project1999.com/Atramentous_Shield or
https://wiki.project1999.com/Sarnak_Battle_Shield until you get a raid shield.