OracleCommand SQL Parameters Binding

Oracle has a different syntax for parameters than Sql-Server. So use : instead of @

using(var con=new OracleConnection(connectionString))
{
   con.open();
   var sql = "insert into users values (:id,:name,:surname,:username)";

   using(var cmd = new OracleCommand(sql,con)
   {
      OracleParameter[] parameters = new OracleParameter[] {
             new OracleParameter("id",1234),
             new OracleParameter("name","John"),
             new OracleParameter("surname","Doe"),
             new OracleParameter("username","johnd")
      };

      cmd.Parameters.AddRange(parameters);
      cmd.ExecuteNonQuery();
   }
}

When using named parameters in an OracleCommand you must precede the parameter name with a colon (:).

http://msdn.microsoft.com/en-us/library/system.data.oracleclient.oraclecommand.parameters.aspx

Remove single quotes around @username, and with respect to oracle use : with parameter name instead of @, like:

OracleCommand oraCommand = new OracleCommand("SELECT fullname FROM sup_sys.user_profile
                           WHERE domain_user_name = :userName", db);
oraCommand.Parameters.Add(new OracleParameter("userName", domainUser));

Source: Using Parameters

You need to use something like this:

 OracleCommand oraCommand = new OracleCommand("SELECT fullname FROM sup_sys.user_profile
                       WHERE domain_user_name = :userName", db);

More can be found in this MSDN article: http://msdn.microsoft.com/en-us/library/system.data.oracleclient.oraclecommand.parameters%28v=vs.100%29.aspx

It is advised you use the : character instead of @ for Oracle.