With current state of AoS, it is almost no point to try increasing Oldblood's survivability. The best thing I can think of is Malign Sorcery book artefacts.
Doppelganger cloak from Ulgu allows once per game to make it impossible to target bearer with melee attacks until he attacks himself. This is the best defensive artefact, IMO, becaue it ensures that Oldblood will attack first before being taken out. Other good defescive artefact is Miasmatic Blade, which gives bearer -1 to-hit on him. With summon starlight it could be -2 to-hit. But, tbh, most stuff will still butcher the carno.
I prefere to use him as suicidal hero. His command ability is very strong and he can take down units which are way more expensive. You can still use malign sorcery artefacts to boost his attack (Dimensional blade makes his spear Rend -3 which will negate most saves), but you I find Doppelganger cloack still better, since it ensures that Oldblood stikes before beign removed. Use Lords of Space and Time to bring Oldblood in position and than use his command ability multiple times. He can 10-11 attacks with his jaw or spear will devastate most stuff. He is best to be used in Dracothion's tail battalion, since he could be put in reserves and placed anywhere on battlefield turn 1 or 2. Dracothion's tail also gives a lot of command points, so Oldblood's command ability can be used 5-6 times, giving him 10-12 bonus attacks.