diff --git a/encoding/protobuf/parse.go b/encoding/protobuf/parse.go
index ad49a04..8593aa3 100644
--- a/encoding/protobuf/parse.go
+++ b/encoding/protobuf/parse.go
@@ -644,8 +644,7 @@
 		}
 		switch x := v.(type) {
 		case *proto.OneOfField:
-			f := p.parseField(s, 0, x.Field)
-			f.Optional = token.NoPos
+			p.parseField(s, 0, x.Field)
 
 		default:
 			p.messageField(s, 1, v)
diff --git a/encoding/protobuf/testdata/attributes.proto.out.cue b/encoding/protobuf/testdata/attributes.proto.out.cue
index 73f70ad..0bdd410 100644
--- a/encoding/protobuf/testdata/attributes.proto.out.cue
+++ b/encoding/protobuf/testdata/attributes.proto.out.cue
@@ -62,28 +62,28 @@
 //  The attribute value.
 Attributes_AttributeValue: {
 	//  Used for values of type STRING, DNS_NAME, EMAIL_ADDRESS, and URI
-	stringValue: string @protobuf(2,name=string_value)
+	stringValue?: string @protobuf(2,name=string_value)
 } | {
 	//  Used for values of type INT64
-	int64Value: int64 @protobuf(3,name=int64_value)
+	int64Value?: int64 @protobuf(3,name=int64_value)
 } | {
 	//  Used for values of type DOUBLE
-	doubleValue: float64 @protobuf(4,type=double,name=double_value)
+	doubleValue?: float64 @protobuf(4,type=double,name=double_value)
 } | {
 	//  Used for values of type BOOL
-	boolValue: bool @protobuf(5,name=bool_value)
+	boolValue?: bool @protobuf(5,name=bool_value)
 } | {
 	//  Used for values of type BYTES
-	bytesValue: bytes @protobuf(6,name=bytes_value)
+	bytesValue?: bytes @protobuf(6,name=bytes_value)
 } | {
 	//  Used for values of type TIMESTAMP
-	timestampValue: time.Time @protobuf(7,type=google.protobuf.Timestamp,name=timestamp_value)
+	timestampValue?: time.Time @protobuf(7,type=google.protobuf.Timestamp,name=timestamp_value)
 } | {
 	//  Used for values of type DURATION
-	durationValue: time.Duration @protobuf(8,type=google.protobuf.Duration,name=duration_value)
+	durationValue?: time.Duration @protobuf(8,type=google.protobuf.Duration,name=duration_value)
 } | {
 	//  Used for values of type STRING_MAP
-	stringMapValue: Attributes_StringMap @protobuf(9,type=StringMap,name=string_map_value)
+	stringMapValue?: Attributes_StringMap @protobuf(9,type=StringMap,name=string_map_value)
 }
 
 //  Defines a string map.
diff --git a/encoding/protobuf/testdata/istio.io/api/mixer/v1/attributes_proto_gen.cue b/encoding/protobuf/testdata/istio.io/api/mixer/v1/attributes_proto_gen.cue
index 73f70ad..0bdd410 100644
--- a/encoding/protobuf/testdata/istio.io/api/mixer/v1/attributes_proto_gen.cue
+++ b/encoding/protobuf/testdata/istio.io/api/mixer/v1/attributes_proto_gen.cue
@@ -62,28 +62,28 @@
 //  The attribute value.
 Attributes_AttributeValue: {
 	//  Used for values of type STRING, DNS_NAME, EMAIL_ADDRESS, and URI
-	stringValue: string @protobuf(2,name=string_value)
+	stringValue?: string @protobuf(2,name=string_value)
 } | {
 	//  Used for values of type INT64
-	int64Value: int64 @protobuf(3,name=int64_value)
+	int64Value?: int64 @protobuf(3,name=int64_value)
 } | {
 	//  Used for values of type DOUBLE
-	doubleValue: float64 @protobuf(4,type=double,name=double_value)
+	doubleValue?: float64 @protobuf(4,type=double,name=double_value)
 } | {
 	//  Used for values of type BOOL
-	boolValue: bool @protobuf(5,name=bool_value)
+	boolValue?: bool @protobuf(5,name=bool_value)
 } | {
 	//  Used for values of type BYTES
-	bytesValue: bytes @protobuf(6,name=bytes_value)
+	bytesValue?: bytes @protobuf(6,name=bytes_value)
 } | {
 	//  Used for values of type TIMESTAMP
-	timestampValue: time.Time @protobuf(7,type=google.protobuf.Timestamp,name=timestamp_value)
+	timestampValue?: time.Time @protobuf(7,type=google.protobuf.Timestamp,name=timestamp_value)
 } | {
 	//  Used for values of type DURATION
-	durationValue: time.Duration @protobuf(8,type=google.protobuf.Duration,name=duration_value)
+	durationValue?: time.Duration @protobuf(8,type=google.protobuf.Duration,name=duration_value)
 } | {
 	//  Used for values of type STRING_MAP
-	stringMapValue: Attributes_StringMap @protobuf(9,type=StringMap,name=string_map_value)
+	stringMapValue?: Attributes_StringMap @protobuf(9,type=StringMap,name=string_map_value)
 }
 
 //  Defines a string map.
diff --git a/encoding/protobuf/testdata/istio.io/api/mixer/v1/config/client/api_spec_proto_gen.cue b/encoding/protobuf/testdata/istio.io/api/mixer/v1/config/client/api_spec_proto_gen.cue
index 3650de8..b5afcbe 100644
--- a/encoding/protobuf/testdata/istio.io/api/mixer/v1/config/client/api_spec_proto_gen.cue
+++ b/encoding/protobuf/testdata/istio.io/api/mixer/v1/config/client/api_spec_proto_gen.cue
@@ -126,7 +126,7 @@
 	//      /dictionary/{term:1}/{term}
 	//      /search{?q*,lang}
 	// 
-	uriTemplate: string @protobuf(3,name=uri_template)
+	uriTemplate?: string @protobuf(3,name=uri_template)
 } | {
 	//  EXPERIMENTAL:
 	// 
@@ -136,7 +136,7 @@
 	// 
 	//      "^/pets/(.*?)?"
 	// 
-	regex: string @protobuf(4)
+	regex?: string @protobuf(4)
 }
 
 //  APIKey defines the explicit configuration for generating the
@@ -155,7 +155,7 @@
 	// 
 	//      GET /something?api_key=abcdef12345
 	// 
-	query: string @protobuf(1)
+	query?: string @protobuf(1)
 } | {
 	//  API key is sent in a request header. `header` represents the
 	//  header name.
@@ -166,7 +166,7 @@
 	//      GET /something HTTP/1.1
 	//      X-API-Key: abcdef12345
 	// 
-	header: string @protobuf(2)
+	header?: string @protobuf(2)
 } | {
 	//  API key is sent in a
 	//  [cookie](https://swagger.io/docs/specification/authentication/cookie-authentication),
@@ -177,7 +177,7 @@
 	//      GET /something HTTP/1.1
 	//      Cookie: X-API-KEY=abcdef12345
 	// 
-	cookie: string @protobuf(3)
+	cookie?: string @protobuf(3)
 }
 
 //  HTTPAPISpecReference defines a reference to an HTTPAPISpec. This is
diff --git a/encoding/protobuf/testdata/istio.io/api/mixer/v1/config/client/quota_proto_gen.cue b/encoding/protobuf/testdata/istio.io/api/mixer/v1/config/client/quota_proto_gen.cue
index e5c97c8..e0a314b 100644
--- a/encoding/protobuf/testdata/istio.io/api/mixer/v1/config/client/quota_proto_gen.cue
+++ b/encoding/protobuf/testdata/istio.io/api/mixer/v1/config/client/quota_proto_gen.cue
@@ -76,13 +76,13 @@
 }
 StringMatch: {
 	//  exact string match
-	exact: string @protobuf(1)
+	exact?: string @protobuf(1)
 } | {
 	//  prefix-based match
-	prefix: string @protobuf(2)
+	prefix?: string @protobuf(2)
 } | {
 	//  ECMAscript style regex-based match
-	regex: string @protobuf(3)
+	regex?: string @protobuf(3)
 }
 
 //  Specifies a match clause to match Istio attributes