数据库事务怎么结束？

我用VS2010做一个Flash报表，想把数据库里查到的表放到一个数组中，在转化的过程中总是提示 ‘SET TRANSACTION READ ONLY’必须是事务执行的第一句，在网上查了下，大体就是说有没结束的sql语句之类，还有提到commit的，但是不知道具体怎么操作，望懂的朋友指点一二，本人菜鸟，见笑啦~
出错代码
public static DataTable ReturnDataTable(string sql, OracleParameter[] op)
        {
            OracleConnection conn = new OracleConnection(ConnectionString);
            DataTable dt = new DataTable();
            try
            {
                OracleCommand cmd = new OracleCommand();
                cmd.Connection = conn;
                conn.Close();
                conn.Open();
                cmd.Transaction = conn.BeginTransaction();
                = "SET TRANSACTION READ ONLY";
                OracleDataReader reader = cmd.ExecuteReader();//到此跳转出错
                reader.Close();
                = sql;
                cmd.Parameters.AddRange(op);
                OracleDataAdapter ad = new OracleDataAdapter(cmd);
                ad.Fill(dt);
                conn.Close();
            }
            catch (Exception ex)
            {
                conn.Close();
                throw new Exception(ex.Message);
            }
            return dt;
        }