From the information given it seems that the hot water system is a common property issue. If the plumber has advised he can fix it and the OC has refused, this is appalling, someone could get seriously hurt and the OC could be held liable.
We installed a new hot water system not long ago, with tempering valves going into each lot, which we didn’t have before due to the age of the building. I don’t think a valve would fix this.