diff --git a/django/db/backends/base/features.py b/django/db/backends/base/features.py
index 4fd21beee3c..5c99736f223 100644
--- a/django/db/backends/base/features.py
+++ b/django/db/backends/base/features.py
@@ -235,6 +235,7 @@ class BaseDatabaseFeatures:
     supports_select_difference = True
     supports_slicing_ordering_in_compound = False
     supports_parentheses_in_compound = True
+    requires_compound_order_by_subquery = False
 
     # Does the database support SQL 2003 FILTER (WHERE ...) in aggregate
     # expressions?
diff --git a/django/db/backends/oracle/features.py b/django/db/backends/oracle/features.py
index 49e58ff59df..9a98616dc21 100644
--- a/django/db/backends/oracle/features.py
+++ b/django/db/backends/oracle/features.py
@@ -69,6 +69,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     supports_partial_indexes = False
     can_rename_index = True
     supports_slicing_ordering_in_compound = True
+    requires_compound_order_by_subquery = True
     allows_multiple_constraints_on_same_fields = False
     supports_boolean_expr_in_select_clause = False
     supports_comparing_boolean_expr = False
diff --git a/django/db/models/sql/compiler.py b/django/db/models/sql/compiler.py
index 4668a820fbc..f566546307d 100644
--- a/django/db/models/sql/compiler.py
+++ b/django/db/models/sql/compiler.py
@@ -435,21 +435,18 @@ class SQLCompiler:
 
         for expr, is_ref in self._order_by_pairs():
             resolved = expr.resolve_expression(self.query, allow_joins=True, reuse=None)
-            if self.query.combinator and self.select:
+            if not is_ref and self.query.combinator and self.select:
                 src = resolved.expression
                 expr_src = expr.expression
-                # Relabel order by columns to raw numbers if this is a combined
-                # query; necessary since the columns can't be referenced by the
-                # fully qualified name and the simple column names may collide.
-                for idx, (sel_expr, _, col_alias) in enumerate(self.select):
-                    if is_ref and col_alias == src.refs:
-                        src = src.source
-                    elif col_alias and not (
+                for sel_expr, _, col_alias in self.select:
+                    if col_alias and not (
                         isinstance(expr_src, F) and col_alias == expr_src.name
                     ):
                         continue
                     if src == sel_expr:
-                        resolved.set_source_expressions([RawSQL("%d" % (idx + 1), ())])
+                        resolved.set_source_expressions(
+                            [Ref(col_alias if col_alias else src.target.column, src)]
+                        )
                         break
                 else:
                     if col_alias:
@@ -853,7 +850,11 @@ class SQLCompiler:
                 for _, (o_sql, o_params, _) in order_by:
                     ordering.append(o_sql)
                     params.extend(o_params)
-                result.append("ORDER BY %s" % ", ".join(ordering))
+                order_by_sql = "ORDER BY %s" % ", ".join(ordering)
+                if combinator and features.requires_compound_order_by_subquery:
+                    result = ["SELECT * FROM (", *result, ")", order_by_sql]
+                else:
+                    result.append(order_by_sql)
 
             if with_limit_offset:
                 result.append(
diff --git a/tests/queries/test_qs_combinators.py b/tests/queries/test_qs_combinators.py
index 3cd19d5f314..5fc09ca9225 100644
--- a/tests/queries/test_qs_combinators.py
+++ b/tests/queries/test_qs_combinators.py
@@ -61,6 +61,24 @@ class QuerySetSetOperationTests(TestCase):
         self.assertSequenceEqual(qs3.none(), [])
         self.assertNumbersEqual(qs3, [0, 1, 8, 9], ordered=False)
 
+    def test_union_order_with_null_first_last(self):
+        Number.objects.filter(other_num=5).update(other_num=None)
+        qs1 = Number.objects.filter(num__lte=1)
+        qs2 = Number.objects.filter(num__gte=2)
+        qs3 = qs1.union(qs2)
+        self.assertSequenceEqual(
+            qs3.order_by(
+                F("other_num").asc(nulls_first=True),
+            ).values_list("other_num", flat=True),
+            [None, 1, 2, 3, 4, 6, 7, 8, 9, 10],
+        )
+        self.assertSequenceEqual(
+            qs3.order_by(
+                F("other_num").asc(nulls_last=True),
+            ).values_list("other_num", flat=True),
+            [1, 2, 3, 4, 6, 7, 8, 9, 10, None],
+        )
+
     @skipUnlessDBFeature("supports_select_intersection")
     def test_intersection_with_empty_qs(self):
         qs1 = Number.objects.all()