diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index c8acd3d..c3e487b 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -1,6 +1,11 @@
 Changelog
 =========
 
+UNRELEASED
+----------
+
+* Fix OID encoding/decoding for the first octet according to ITU-T X.690
+
 2.7.0 (2023-01-17)
 ------------------
 
diff --git a/src/asn1.py b/src/asn1.py
index 4d59205..b8b6919 100644
--- a/src/asn1.py
+++ b/src/asn1.py
@@ -388,7 +388,7 @@ def _encode_object_identifier(self, oid):  # type: (str) -> bytes
         if not self._re_oid.match(oid):
             raise Error('Illegal object identifier')
         cmps = list(map(int, oid.split('.')))
-        if cmps[0] > 39 or cmps[1] > 39:
+        if (cmps[0] <= 1 and cmps[1] > 39) or cmps[0] > 2:
             raise Error('Illegal object identifier')
         cmps = [40 * cmps[0] + cmps[1]] + cmps[2:]
         cmps.reverse()
@@ -686,9 +686,12 @@ def _decode_object_identifier(bytes_data):  # type: (bytes) -> str
             if not byte & 0x80:
                 result.append(value)
                 value = 0
-        if len(result) == 0 or result[0] > 1599:
+        if len(result) == 0:
             raise Error('ASN1 syntax error')
-        result = [result[0] // 40, result[0] % 40] + result[1:]
+        if result[0] // 40 <= 1:
+            result = [result[0] // 40, result[0] % 40] + result[1:]
+        else:
+            result = [2, result[0] - 80] + result[1:]
         result = list(map(str, result))
         return str('.'.join(result))
 
diff --git a/tests/test_asn1.py b/tests/test_asn1.py
index e5ce2e8..c0134a6 100644
--- a/tests/test_asn1.py
+++ b/tests/test_asn1.py
@@ -177,10 +177,14 @@ def test_object_identifier(self):
     def test_long_object_identifier(self):
         enc = asn1.Encoder()
         enc.start()
-        enc.write('39.2.3', asn1.Numbers.ObjectIdentifier)
+        enc.write('2.1482.3', asn1.Numbers.ObjectIdentifier)
         res = enc.output()
         assert res == b'\x06\x03\x8c\x1a\x03'
         enc.start()
+        enc.write('2.999.3', asn1.Numbers.ObjectIdentifier)
+        res = enc.output()
+        assert res == b'\x06\x03\x88\x37\x03'
+        enc.start()
         enc.write('1.39.3', asn1.Numbers.ObjectIdentifier)
         res = enc.output()
         assert res == b'\x06\x02\x4f\x03'
@@ -341,7 +345,7 @@ def test_error_object_identifier(self):
         enc = asn1.Encoder()
         enc.start()
         pytest.raises(asn1.Error, enc.write, '1', asn1.Numbers.ObjectIdentifier)
-        pytest.raises(asn1.Error, enc.write, '40.2.3', asn1.Numbers.ObjectIdentifier)
+        pytest.raises(asn1.Error, enc.write, '3.2.3', asn1.Numbers.ObjectIdentifier)
         pytest.raises(asn1.Error, enc.write, '1.40.3', asn1.Numbers.ObjectIdentifier)
         pytest.raises(asn1.Error, enc.write, '1.2.3.', asn1.Numbers.ObjectIdentifier)
         pytest.raises(asn1.Error, enc.write, '.1.2.3', asn1.Numbers.ObjectIdentifier)
@@ -521,7 +525,11 @@ def test_long_object_identifier(self):
         buf = b'\x06\x03\x8c\x1a\x03'
         dec.start(buf)
         tag, val = dec.read()
-        assert val == u'39.2.3'
+        assert val == u'2.1482.3'
+        buf = b'\x06\x03\x88\x37\x03'
+        dec.start(buf)
+        tag, val = dec.read()
+        assert val == u'2.999.3'
         buf = b'\x06\x02\x4f\x03'
         dec.start(buf)
         tag, val = dec.read()
@@ -745,13 +753,7 @@ def test_error_non_normalized_negative_integer(self):
         pytest.raises(asn1.Error, dec.read)
 
     def test_error_non_normalised_object_identifier(self):
-        buf = b'\x06\x02\x80\x01'
-        dec = asn1.Decoder()
-        dec.start(buf)
-        pytest.raises(asn1.Error, dec.read)
-
-    def test_error_object_identifier_with_too_large_first_component(self):
-        buf = b'\x06\x02\x8c\x40'
+        buf = b'\x06\x02\x01\x80'
         dec = asn1.Decoder()
         dec.start(buf)
         pytest.raises(asn1.Error, dec.read)
@@ -1079,12 +1081,22 @@ def test_octet_string(self):
     def test_null(self):
         TestEncoderDecoder.assert_encode_decode(None, asn1.Numbers.Null)
 
-    def test_object_identifier(self):
+    def test_real_object_identifier(self):
         TestEncoderDecoder.assert_encode_decode(
             '1.2.840.113554.1.2.1.1',
             asn1.Numbers.ObjectIdentifier
         )
 
+    def test_long_object_identifier(self):
+        for v in \
+        (
+            '2.60.3',
+            '2.999.3',
+            '1.39.3',
+            '1.2.300000'
+        ):
+            TestEncoderDecoder.assert_encode_decode(v, asn1.Numbers.ObjectIdentifier)
+
     def test_enumerated(self):
         for v in (1, 2, 42):
             TestEncoderDecoder.assert_encode_decode(v, asn1.Numbers.Enumerated)