diff --git a/src/main/scala/scalapb/json4s/JsonFormat.scala b/src/main/scala/scalapb/json4s/JsonFormat.scala
index 8723049..dd4ad60 100644
--- a/src/main/scala/scalapb/json4s/JsonFormat.scala
+++ b/src/main/scala/scalapb/json4s/JsonFormat.scala
@@ -461,6 +461,7 @@ class Printer private (config: Printer.PrinterConfig) {
 object Parser {
   private final case class ParserConfig(
       isIgnoringUnknownFields: Boolean,
+      isIgnoringOverlappingOneofFields: Boolean,
       mapEntriesAsKeyValuePairs: Boolean,
       formatRegistry: FormatRegistry,
       typeRegistry: TypeRegistry
@@ -472,6 +473,7 @@ class Parser private (config: Parser.ParserConfig) {
     this(
       Parser.ParserConfig(
         isIgnoringUnknownFields = false,
+        isIgnoringOverlappingOneofFields = false,
         mapEntriesAsKeyValuePairs = false,
         JsonFormat.DefaultRegistry,
         TypeRegistry.empty
@@ -490,6 +492,7 @@ class Parser private (config: Parser.ParserConfig) {
     this(
       Parser.ParserConfig(
         isIgnoringUnknownFields = false,
+        isIgnoringOverlappingOneofFields = false,
         mapEntriesAsKeyValuePairs = false,
         formatRegistry,
         typeRegistry
@@ -499,6 +502,9 @@ class Parser private (config: Parser.ParserConfig) {
   def ignoringUnknownFields: Parser =
     new Parser(config.copy(isIgnoringUnknownFields = true))
 
+  def ignoringOverlappingOneofFields: Parser =
+    new Parser(config.copy(isIgnoringOverlappingOneofFields = true))
+
   def mapEntriesAsKeyValuePairs: Parser =
     new Parser(config.copy(mapEntriesAsKeyValuePairs = true))
 
@@ -593,12 +599,23 @@ class Parser private (config: Parser.ParserConfig) {
       case None =>
         value match {
           case JObject(fields) =>
+            val usedOneofs = mutable.Set[OneofDescriptor]()
             val fieldMap = JsonFormat.MemorizedFieldNameMap(cmp.scalaDescriptor)
             val valueMapBuilder = Map.newBuilder[FieldDescriptor, PValue]
             fields.foreach { case (name: String, jValue: JValue) =>
               if (fieldMap.contains(name)) {
                 if (jValue != JNull) {
                   val fd = fieldMap(name)
+                  fd.containingOneof.foreach(o =>
+                    if (
+                      !config.isIgnoringOverlappingOneofFields && !usedOneofs
+                        .add(o)
+                    ) {
+                      throw new JsonFormatException(
+                        s"Overlapping field '${name}' in oneof"
+                      )
+                    }
+                  )
                   valueMapBuilder += (fd -> parseValue(fd, jValue))
                 }
               } else if (
diff --git a/src/test/scala/scalapb/json4s/JsonFormatSpec.scala b/src/test/scala/scalapb/json4s/JsonFormatSpec.scala
index 529578a..41a3fd4 100644
--- a/src/test/scala/scalapb/json4s/JsonFormatSpec.scala
+++ b/src/test/scala/scalapb/json4s/JsonFormatSpec.scala
@@ -7,6 +7,7 @@ import com.google.protobuf.any.{Any => PBAny}
 import com.google.protobuf.util.JsonFormat.{TypeRegistry => JavaTypeRegistry}
 import com.google.protobuf.util.{JsonFormat => JavaJsonFormat}
 import jsontest.custom_collection.{Guitar, Studio}
+import jsontest.oneof.OneOf
 import jsontest.test._
 import jsontest.test3._
 import org.json4s.JsonDSL._
@@ -674,6 +675,20 @@ class JsonFormatSpec
     JsonFormat.parser.fromJsonString[TestAllTypes](javaJson) must be(obj)
   }
 
+  "oneofs" should "fail for overlapping keys if failOnOverlappingOneofKeys" in new DefaultParserContext {
+    val extraKey = """{"primitive": "", "wrapper": ""}"""
+    assertFails(extraKey, OneOf)
+  }
+
+  "oneofs" should "not fail for overlapping keys if ignoreOverlappingOneofKeys" in {
+    val extraKey = """{"primitive": "", "wrapper": ""}"""
+    val scalaParser = new Parser().ignoringOverlappingOneofFields
+    val parsedScala = scalaParser.fromJsonString[OneOf](extraKey)(OneOf)
+    parsedScala must be(
+      OneOf(field = OneOf.Field.Primitive(""))
+    )
+  }
+
   "TestProto" should "be TestJsonWithMapEntriesAsKeyValuePairs when converted to Proto with mapEntriesAsKeyValuePairs setting" in {
     JsonFormat.printer.mapEntriesAsKeyValuePairs.toJson(TestProto) must be(
       parse(TestJsonWithMapEntriesAsKeyValuePairs)