diff --git a/include/mbedtls/ecp.h b/include/mbedtls/ecp.h
index badfe86..32c4528 100644
--- a/include/mbedtls/ecp.h
+++ b/include/mbedtls/ecp.h
@@ -504,8 +504,14 @@
 #if defined(MBEDTLS_ASN1_WRITE_C) && defined(MBEDTLS_OID_C)
 /**
  * \brief           Maximum size of the output of mbedtls_ecp_ansi_write_group
+ *
+ * \note            The maximum size of the OID of a supported group + 2 for
+ *                  tag and length. Maximum size 30 is based on the length of
+ *                  the OID for primeCurves 10-38 over GF(p) defined by the
+ *                  CDC Group, as they seem to have the longest OID out of
+ *                  curves in use.
  */
-#define MBEDTLS_OID_EC_GRP_MAX_SIZE 12
+#define MBEDTLS_ECP_GRP_OID_MAX_SIZE ( 30 + 2 )
 
 /**
  * \brief           Write the ANSI X9.62/RFC5480 OID ECParameters of a group
diff --git a/include/mbedtls/oid.h b/include/mbedtls/oid.h
index fcecdaf..618c0ea 100644
--- a/include/mbedtls/oid.h
+++ b/include/mbedtls/oid.h
@@ -282,6 +282,8 @@
 
 /*
  * ECParameters namedCurve identifiers, from RFC 5480, RFC 5639, and SEC2
+ * When adding new OID's, please update \c MBEDTLS_ECP_GRP_OID_MAX_SIZE
+ * in ecp.h
  */
 
 /* secp192r1 OBJECT IDENTIFIER ::= {