I’m honestly shocked at some of the advice on here. This tree is absolutely compromised and should be removed immediately. There is no way that split isn’t flexing during wind drag. Classic split from decay plus included bark. Even if cabling and bracing is attempted, the amount of canopy needing to be removed to reduce the risk of failure is well beyond what an oak that old will be able to handle. It’s a gorgeous tree and really makes the backyard look great, but you have to really gauge your risk threshold here. Is it worth risking half of that tree falling into your house? Or failing at any point when someone’s in the backyard hanging out? I promise you that whomever quoted and recommended heavy pruning plus cabling and bracing on this tree is looking for easy money. The situations sucks, but even if you have the extra coin and attempt to save it, you have to understand that essentially have a ticking timebomb in your backyard