It's rather complex. Some employer pension plans do provide a bridge to old age security for workers who retire on a company pension before age 65. I don't have the figures on how common that is, but it's not uncommon. So there will be some cost to employer pension plans in raising the age of eligibility for OAS.
The vast majority of pension plans in Canada are integrated with the Canada pension plan, meaning that they provide a defined benefit on top of what CPP provides. That's less often the case with OAS, but there is that bridge provision there.