use this code
Example 1
$stallFirstSel=oci_parse($conn, 'SELECT * FROM ACTIVIDAD');
oci_execute($stallFirstSel);
echo "<form id='formActis' method='POST' action='reserves_es.php'><table><tr>";
echo "<td>";
echo "<select name='selActi[]'>";
while (($row=oci_fetch_array($stallFirstSel,OCI_BOTH))!=false){
echo "<option id='optActi'>".$row['NOM']."</option>";
}
echo "</select>";
echo "</td><td><input type='submit' value='Enviar'></td>";
echo " </tr></table></form>";
Example 2
$stallFirstSel=oci_parse($conn, 'SELECT * FROM ACTIVIDAD');
oci_execute($stallFirstSel);
echo "<table><tr>";
echo "<td><form id='formActis' method='POST' action='reserves_es.php'>";
echo "<select name='selActi[]'>";
while (($row=oci_fetch_array($stallFirstSel,OCI_BOTH))!=false){
echo "<option id='optActi'>".$row['NOM']."</option>";
}
echo "</select>";
echo " <input type='submit' value='Enviar'></form></td>";
echo " </tr></table>";