diff --git a/generator/nanopb_generator.py b/generator/nanopb_generator.py
index 07beed3..91c8975 100755
--- a/generator/nanopb_generator.py
+++ b/generator/nanopb_generator.py
@@ -362,6 +362,7 @@
         self.fixed_count = False
         self.callback_datatype = field_options.callback_datatype
         self.math_include_required = False
+        self.sort_by_tag = field_options.sort_by_tag
 
         if field_options.type == nanopb_pb2.FT_INLINE:
             # Before nanopb-0.3.8, fixed length bytes arrays were specified
@@ -871,7 +872,7 @@
 # ---------------------------------------------------------------------------
 
 class OneOf(Field):
-    def __init__(self, struct_name, oneof_desc):
+    def __init__(self, struct_name, oneof_desc, oneof_options):
         self.struct_name = struct_name
         self.name = oneof_desc.name
         self.ctype = 'union'
@@ -880,7 +881,8 @@
         self.allocation = 'ONEOF'
         self.default = None
         self.rules = 'ONEOF'
-        self.anonymous = False
+        self.anonymous = oneof_options.anonymous_oneof
+        self.sort_by_tag = oneof_options.sort_by_tag
         self.has_msg_cb = False
 
     def add_field(self, field):
@@ -888,7 +890,9 @@
         field.rules = 'ONEOF'
         field.anonymous = self.anonymous
         self.fields.append(field)
-        self.fields.sort(key = lambda f: f.tag)
+
+        if self.sort_by_tag:
+            self.fields.sort()
 
         if field.pbtype == 'MSG_W_CB':
             self.has_msg_cb = True
@@ -1019,11 +1023,8 @@
                 elif oneof_options.type == nanopb_pb2.FT_IGNORE:
                     pass # No union and skip fields also
                 else:
-                    oneof = OneOf(self.name, f)
-                    if oneof_options.anonymous_oneof:
-                        oneof.anonymous = True
+                    oneof = OneOf(self.name, f, oneof_options)
                     self.oneofs[i] = oneof
-                    self.fields.append(oneof)
         else:
             sys.stderr.write('Note: This Python protobuf library has no OneOf support\n')
 
@@ -1038,6 +1039,9 @@
                 f.oneof_index not in no_unions):
                 if f.oneof_index in self.oneofs:
                     self.oneofs[f.oneof_index].add_field(field)
+
+                    if self.oneofs[f.oneof_index] not in self.fields:
+                        self.fields.append(self.oneofs[f.oneof_index])
             else:
                 self.fields.append(field)
                 if field.math_include_required:
@@ -1049,6 +1053,9 @@
             if field_options.type != nanopb_pb2.FT_IGNORE:
                 self.fields.append(ExtensionRange(self.name, range_start, field_options))
 
+        if message_options.sort_by_tag:
+            self.fields.sort()
+
     def get_dependencies(self):
         '''Get list of type names that this structure refers to.'''
         deps = []
@@ -1064,7 +1071,7 @@
             # Therefore add a dummy field if an empty message occurs.
             result += '    char dummy_field;'
 
-        result += '\n'.join([str(f) for f in sorted(self.fields)])
+        result += '\n'.join([str(f) for f in self.fields])
 
         if Globals.protoc_insertion_points:
             result += '\n/* @@protoc_insertion_point(struct:%s) */' % self.name
@@ -1090,7 +1097,7 @@
             return '{0}'
 
         parts = []
-        for field in sorted(self.fields):
+        for field in self.fields:
             parts.append(field.get_initializer(null_init))
         return '{' + ', '.join(parts) + '}'
 
diff --git a/generator/proto/nanopb.proto b/generator/proto/nanopb.proto
index 6a3c620..96e24b4 100644
--- a/generator/proto/nanopb.proto
+++ b/generator/proto/nanopb.proto
@@ -141,6 +141,11 @@
   
   // Override type of the field in generated C code. Only to be used with related field types
   optional google.protobuf.FieldDescriptorProto.Type type_override = 27;
+
+  // Due to historical reasons, nanopb orders fields in structs by their tag number
+  // instead of the order in .proto. Set this to false to keep the .proto order.
+  // The default value will probably change to false in nanopb-0.5.0.
+  optional bool sort_by_tag = 28 [default = true];
 }
 
 // Extensions to protoc 'Descriptor' type in order to define options
diff --git a/tests/sort_by_tag/SConscript b/tests/sort_by_tag/SConscript
new file mode 100644
index 0000000..96a746d
--- /dev/null
+++ b/tests/sort_by_tag/SConscript
@@ -0,0 +1,8 @@
+# Test sort_by_tag generator option
+
+Import("env")
+
+env.NanopbProto(["sort_by_tag.proto", "sort_by_tag.options"])
+test = env.Program(["sort_by_tag.c", "sort_by_tag.pb.c", "$COMMON/pb_encode.o", "$COMMON/pb_decode.o", "$COMMON/pb_common.o"])
+env.RunTest(test)
+
diff --git a/tests/sort_by_tag/sort_by_tag.c b/tests/sort_by_tag/sort_by_tag.c
new file mode 100644
index 0000000..bdb1e3e
--- /dev/null
+++ b/tests/sort_by_tag/sort_by_tag.c
@@ -0,0 +1,55 @@
+#include "sort_by_tag.pb.h"
+#include <pb_encode.h>
+#include <pb_decode.h>
+#include "unittests.h"
+
+int main()
+{
+    int status = 0;
+    size_t msglen;
+    pb_byte_t buf[256];
+    
+    {
+        pb_ostream_t ostream = pb_ostream_from_buffer(buf, sizeof(buf));
+        Unsorted msg = Unsorted_init_zero;
+        COMMENT("Test encoding with unsorted structure");
+
+        TEST(&msg.first < &msg.oneof.second);
+        TEST(&msg.oneof.second < &msg.last);
+
+        msg.first = 101;
+        msg.which_oneof = Unsorted_second_tag;
+        msg.oneof.second = 102;
+        msg.last = 103;
+
+        if (!pb_encode(&ostream, Unsorted_fields, &msg))
+        {
+            fprintf(stderr, "Encoding failed: %s\n", PB_GET_ERROR(&ostream));
+            return 1;
+        }
+
+        msglen = ostream.bytes_written;
+    }
+
+    {
+        pb_istream_t istream = pb_istream_from_buffer(buf, msglen);
+        Sorted msg = Sorted_init_zero;
+        COMMENT("Test decoding with sorted structure");
+
+        if (!pb_decode(&istream, Sorted_fields, &msg))
+        {
+            fprintf(stderr, "Decoding failed: %s\n", PB_GET_ERROR(&istream));
+            return 2;
+        }
+
+        TEST(msg.first == 101);
+        TEST(msg.which_oneof == Sorted_second_tag);
+        TEST(msg.oneof.second == 102);
+        TEST(msg.last == 103);
+
+        TEST(&msg.first > &msg.oneof.second);
+        TEST(&msg.oneof.second > &msg.last);
+    }
+
+    return status;
+}
\ No newline at end of file
diff --git a/tests/sort_by_tag/sort_by_tag.options b/tests/sort_by_tag/sort_by_tag.options
new file mode 100644
index 0000000..b9b1d8e
--- /dev/null
+++ b/tests/sort_by_tag/sort_by_tag.options
@@ -0,0 +1,2 @@
+Unsorted	sort_by_tag:false
+Sorted		sort_by_tag:true
diff --git a/tests/sort_by_tag/sort_by_tag.proto b/tests/sort_by_tag/sort_by_tag.proto
new file mode 100644
index 0000000..95596bf
--- /dev/null
+++ b/tests/sort_by_tag/sort_by_tag.proto
@@ -0,0 +1,21 @@
+syntax = "proto3";
+
+message Unsorted
+{
+    uint32 first = 99;
+    oneof oneof {
+        uint32 second = 80;
+        uint32 third = 50;
+    }
+    uint32 last = 1;
+}
+
+message Sorted
+{
+    uint32 first = 99;
+    oneof oneof {
+        uint32 second = 80;
+        uint32 third = 50;
+    }
+    uint32 last = 1;
+}