well, the idea of that code is not to supply null to OleDbType.Date parameter but DBNull.Value
You could change it in more "detailed" way as
if (orderedOn != null)
myComm.Parameters.Add("@OrderDate", OleDbType.Date).Value = orderedOn;
else
myComm.Parameters.Add("@OrderDate", OleDbType.Date).Value = DBNull.Value;