I believe that gas and oil burning appliances venting into a single flue is forbidden in New England. I’m not sure about your jurisdiction but if it accepts / uses NFPA (National Fire Protection Association) and IFGC (International Fuel Gas Code) as the City of New York does, than you should be OK with your installation (I’d call your local building department to verify that).
503.5.7.4 Combination gas- and oil fuel-burning appliances. A listed combination gas- and oil fuel-burning appliance shall be permitted to be connected to a single chimney flue. The chimney flue shall be sized to properly vent the appliance.
There are a couple of other requirements when the combination of oil-fired appliances and gas-fired appliances are vented into the same chimney flue;
If both of the appliances use individual vent connectors (highly recommended) tying them to the chimney flue, the smaller diameter vent connector must be vented into the chimney flue above the larger diameter vent connector.
Their chimney penetrations must be positioned at different levels / you cannot have one directly across from the other.
The chimney flue must be properly sized to handle the maximum load of combined BTU’s from the gas and oil burning appliance, as well as the minimum BTU load when only the smallest appliance is operating on its own.
Please let me know if you have any other questions.