The overvoltage tolerant (OVT) pins can operate in the full voltage range of the device i.e. their levels can be up to 3.6 V irrespective of your other VDDs. So you can hook-up a signal in the 3.6 V level to these pins when all your VDDs are at 1.8 V without damaging the IO or the device.
There is no specific datasheet spec for the OVT pins. All the specs that apply to a GPIO pin applies to these. Except that these pins have the ESD diodes (that limit the voltage to VDDIO) removed - hence there is no IDIODE spec for these pins. This allows them to operate to the full voltage range supported by the device. But do note that any voltage beyond 3.6 V can permanently damage the IO/Device.
Meenakshi Sundaram R