diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index 5fb199407..f9b004cfc 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -78,6 +78,14 @@ _STORAGE_HEATER_CODES = frozenset(range(401, 410))
 _ROOM_HEATER_CATEGORY = 10
 _ROOM_HEATER_CONTROL = 2601
 _SOLID_FUEL_ROOM_HEATER_CODES = frozenset(range(631, 637))
+# Oil room heaters (SAP Table 4a 621-625) — category 10, conservative room-heater
+# control, natural fuel heating oil (RdSAP main_fuel 28).
+_OIL_ROOM_HEATER_CODES = frozenset(range(621, 626))
+_OIL_FUEL = 28
+# Fuel-burning room heaters (solid + oil) share category 10 + the conservative
+# room-heater control; only the natural fuel differs by family. (Distinct from
+# `_ROOM_HEATER_CODES` above, which is the electric-room-heater dual-meter set.)
+_CATEGORY_10_ROOM_HEATER_CODES = _SOLID_FUEL_ROOM_HEATER_CODES | _OIL_ROOM_HEATER_CODES
 
 # Natural-fuel coherence (ADR-0041): where an archetype unambiguously implies a
 # fuel, the overlay drags it so a system-only override is self-coherent on a cert
@@ -182,7 +190,7 @@ def _control_for(code: int) -> Optional[int]:
         return _MANUAL_CHARGE_CONTROL
     if code in _GAS_BOILER_CODES:
         return _FULL_BOILER_CONTROL
-    if code in _SOLID_FUEL_ROOM_HEATER_CODES:
+    if code in _CATEGORY_10_ROOM_HEATER_CODES:
         return _ROOM_HEATER_CONTROL
     return None
 
@@ -191,7 +199,7 @@ def _category_for(code: int) -> Optional[int]:
     """The SAP Table 4a heating category a code implies, where the archetype
     fixes it. Room heaters are category 10 — set so a system-only override reads
     as a room-heater system, not whatever category the replaced system carried."""
-    if code in _SOLID_FUEL_ROOM_HEATER_CODES:
+    if code in _CATEGORY_10_ROOM_HEATER_CODES:
         return _ROOM_HEATER_CATEGORY
     if code in _HEAT_PUMP_CODES:
         return _HEAT_PUMP_CATEGORY
@@ -208,6 +216,8 @@ def _natural_fuel_for(code: int) -> Optional[int]:
         return _ELECTRICITY_FUEL
     if code in _SOLID_FUEL_ROOM_HEATER_CODES:
         return _HOUSE_COAL_FUEL
+    if code in _OIL_ROOM_HEATER_CODES:
+        return _OIL_FUEL
     if code in _COMMUNITY_BOILER_CODES:
         return _COMMUNITY_GAS_FUEL
     return None