diff --git a/schemars/src/make_schema.rs b/schemars/src/make_schema.rs
index 4bb6137..5b5dc35 100644
--- a/schemars/src/make_schema.rs
+++ b/schemars/src/make_schema.rs
@@ -262,6 +262,8 @@ map_impl!(<K: Eq + core::hash::Hash, V, H: core::hash::BuildHasher> MakeSchema f
 
 impl<T: MakeSchema> MakeSchema for Option<T> {
     fn is_referenceable() -> bool {
+        // TODO only really needs to be referenceable with option_nullable enabled.
+        // TODO what if T is Box<U> and U is referenceable?
         T::is_referenceable()
     }
 
@@ -279,10 +281,11 @@ impl<T: MakeSchema> MakeSchema for Option<T> {
             }
         }
         if gen.settings().option_nullable {
-            let mut deref = gen.try_get_schema_object(&schema);
+            let deref = gen.try_get_schema_object(&schema);
             debug_assert!(deref.is_some(), "Could not get schema object: {:?}", schema);
-            if let Some(ref mut schema) = deref {
-                schema.extensions.insert("nullable".to_owned(), json!(true));
+            if let Some(mut deref) = deref {
+                deref.extensions.insert("nullable".to_owned(), json!(true));
+                schema = Schema::Object(deref);
             }
         };
         schema
diff --git a/schemars/src/schema.rs b/schemars/src/schema.rs
index 2ae521c..4c298bc 100644
--- a/schemars/src/schema.rs
+++ b/schemars/src/schema.rs
@@ -4,8 +4,6 @@ use serde::{Deserialize, Serialize};
 use serde_json::Value;
 use std::collections::BTreeMap as Map;
 
-// TODO use serde_json::Map (or some other wrapper) instead of BTreeMap to ensure preserve_order is possible
-
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, MakeSchema)]
 #[serde(untagged)]
 pub enum Schema {
diff --git a/schemars/tests/schema-openapi3.json b/schemars/tests/schema-openapi3.json
new file mode 100644
index 0000000..dac35e0
--- /dev/null
+++ b/schemars/tests/schema-openapi3.json
@@ -0,0 +1,191 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "schemars__schema__Schema",
+  "anyOf": [
+    {
+      "type": "boolean"
+    },
+    {
+      "$ref": "#/components/schemas/schemars__schema__SchemaRef"
+    },
+    {
+      "$ref": "#/components/schemas/schemars__schema__SchemaObject"
+    }
+  ],
+  "definitions": {
+    "core__option__Option_schemars__schema__SingleOrVec_schemars__schema__InstanceType__": {
+      "anyOf": [
+        {
+          "$ref": "#/components/schemas/schemars__schema__InstanceType"
+        },
+        {
+          "type": "array",
+          "items": {
+            "$ref": "#/components/schemas/schemars__schema__InstanceType"
+          }
+        }
+      ],
+      "nullable": true
+    },
+    "core__option__Option_schemars__schema__SingleOrVec_schemars__schema__Schema__": {
+      "anyOf": [
+        {
+          "$ref": "#/components/schemas/schemars__schema__Schema"
+        },
+        {
+          "type": "array",
+          "items": {
+            "$ref": "#/components/schemas/schemars__schema__Schema"
+          }
+        }
+      ],
+      "nullable": true
+    },
+    "schemars__schema__InstanceType": {
+      "enum": [
+        "null",
+        "boolean",
+        "object",
+        "array",
+        "number",
+        "string",
+        "integer"
+      ]
+    },
+    "schemars__schema__Schema": {
+      "anyOf": [
+        {
+          "type": "boolean"
+        },
+        {
+          "$ref": "#/components/schemas/schemars__schema__SchemaRef"
+        },
+        {
+          "$ref": "#/components/schemas/schemars__schema__SchemaObject"
+        }
+      ]
+    },
+    "schemars__schema__SchemaObject": {
+      "properties": {
+        "$id": {
+          "type": "string",
+          "nullable": true
+        },
+        "$schema": {
+          "type": "string",
+          "nullable": true
+        },
+        "allOf": {
+          "type": "array",
+          "items": {
+            "$ref": "#/components/schemas/schemars__schema__Schema"
+          },
+          "nullable": true
+        },
+        "anyOf": {
+          "type": "array",
+          "items": {
+            "$ref": "#/components/schemas/schemars__schema__Schema"
+          },
+          "nullable": true
+        },
+        "definitions": {
+          "type": "object",
+          "additionalProperties": {
+            "$ref": "#/components/schemas/schemars__schema__Schema"
+          }
+        },
+        "description": {
+          "type": "string",
+          "nullable": true
+        },
+        "enum": {
+          "type": "array",
+          "items": {},
+          "nullable": true
+        },
+        "extensions": {
+          "type": "object",
+          "additionalProperties": true
+        },
+        "items": {
+          "$ref": "#/components/schemas/core__option__Option_schemars__schema__SingleOrVec_schemars__schema__Schema__"
+        },
+        "not": {
+          "anyOf": [
+            {
+              "type": "boolean"
+            },
+            {
+              "$ref": "#/components/schemas/schemars__schema__SchemaRef"
+            },
+            {
+              "$ref": "#/components/schemas/schemars__schema__SchemaObject"
+            }
+          ],
+          "nullable": true
+        },
+        "oneOf": {
+          "type": "array",
+          "items": {
+            "$ref": "#/components/schemas/schemars__schema__Schema"
+          },
+          "nullable": true
+        },
+        "properties": {
+          "type": "object",
+          "additionalProperties": {
+            "$ref": "#/components/schemas/schemars__schema__Schema"
+          }
+        },
+        "required": {
+          "type": "array",
+          "items": {
+            "type": "string"
+          },
+          "nullable": true
+        },
+        "title": {
+          "type": "string",
+          "nullable": true
+        },
+        "type": {
+          "$ref": "#/components/schemas/core__option__Option_schemars__schema__SingleOrVec_schemars__schema__InstanceType__"
+        }
+      }
+    },
+    "schemars__schema__SchemaRef": {
+      "properties": {
+        "$ref": {
+          "type": "string"
+        }
+      }
+    },
+    "schemars__schema__SingleOrVec_schemars__schema__InstanceType_": {
+      "anyOf": [
+        {
+          "$ref": "#/components/schemas/schemars__schema__InstanceType"
+        },
+        {
+          "type": "array",
+          "items": {
+            "$ref": "#/components/schemas/schemars__schema__InstanceType"
+          }
+        }
+      ]
+    },
+    "schemars__schema__SingleOrVec_schemars__schema__Schema_": {
+      "anyOf": [
+        {
+          "$ref": "#/components/schemas/schemars__schema__Schema"
+        },
+        {
+          "type": "array",
+          "items": {
+            "$ref": "#/components/schemas/schemars__schema__Schema"
+          }
+        }
+      ]
+    }
+  }
+}
\ No newline at end of file
diff --git a/schemars/tests/test.rs b/schemars/tests/test.rs
index 13e8502..9bb82de 100644
--- a/schemars/tests/test.rs
+++ b/schemars/tests/test.rs
@@ -1,5 +1,5 @@
 use schemars::schema::*;
-use schemars::schema_for;
+use schemars::{gen, schema_for};
 use serde_json::{from_str, to_string_pretty};
 use std::fs;
 
@@ -8,7 +8,7 @@ mod tests {
     use super::*;
 
     #[test]
-    fn schema_matches() -> Result<(), Box<dyn std::error::Error>> {
+    fn schema_matches_default_settings() -> Result<(), Box<dyn std::error::Error>> {
         let expected_json = fs::read_to_string("tests/schema.json")?;
         let expected: Schema = from_str(&expected_json)?;
 
@@ -18,4 +18,21 @@ mod tests {
         assert_eq!(actual, expected, "\n\nGenerated schema did not match saved schema - generated schema has been written to \"tests/schema.actual.json\".");
         Ok(())
     }
+
+    #[test]
+    fn schema_matches_openapi3() -> Result<(), Box<dyn std::error::Error>> {
+        let expected_json = fs::read_to_string("tests/schema-openapi3.json")?;
+        let expected: Schema = from_str(&expected_json)?;
+
+        let actual = gen::SchemaSettings::openapi3()
+            .into_generator()
+            .into_root_schema_for::<Schema>();
+        fs::write(
+            "tests/schema-openapi3.actual.json",
+            to_string_pretty(&actual)?,
+        )?;
+
+        assert_eq!(actual, expected, "\n\nGenerated schema did not match saved schema - generated schema has been written to \"tests/schema-openapi3.actual.json\".");
+        Ok(())
+    }
 }