diff --git a/include/ndn-cpp/encoding/oid.hpp b/include/ndn-cpp/encoding/oid.hpp
index bc0dd25..f217889 100644
--- a/include/ndn-cpp/encoding/oid.hpp
+++ b/include/ndn-cpp/encoding/oid.hpp
@@ -39,20 +39,20 @@
   }
 
   std::string 
-  toString();
+  toString() const;
 
-  bool operator == (const OID& oid)
+  bool operator == (const OID& oid) const
   {
     return equal(oid);
   }
 
-  bool operator != (const OID& oid)
+  bool operator != (const OID& oid) const
   {
     return !equal(oid);
   }
   
 private:
-  bool equal(const OID& oid);
+  bool equal(const OID& oid) const;
 
   std::vector<int> oid_;
 };
diff --git a/src/encoding/oid.cpp b/src/encoding/oid.cpp
index db1fbc3..633d1cf 100644
--- a/src/encoding/oid.cpp
+++ b/src/encoding/oid.cpp
@@ -34,11 +34,11 @@
   }
 }
 
-string OID::toString()
+string OID::toString() const
 {
   ostringstream convert;
   
-  vector<int>::iterator it = oid_.begin();
+  vector<int>::const_iterator it = oid_.begin();
   for(; it < oid_.end(); it++){
     if(it != oid_.begin())
       convert << ".";
@@ -48,7 +48,7 @@
   return convert.str();
 }
 
-bool OID::equal(const OID& oid)
+bool OID::equal(const OID& oid) const
 {
   vector<int>::const_iterator i = oid_.begin();
   vector<int>::const_iterator j = oid.oid_.begin();