public void OnConfirm(object sender, EventArgs e)
{
string confirmValue = Request.Form["confirm_value"];
if (confirmValue == "Yes")
{
    //Your logic for OK button
            ScriptManager.RegisterStartupScript(this, this.GetType(), "alert", "alert('you clicked yes'); OpenConfirmDialog();", true);
}
else
{
    //Your logic for cancel button
            ScriptManager.RegisterStartupScript(this, this.GetType(), "alert", "alert('you clicked no'); OpenConfirmDialog();", true);
}
}