Código PHP:
<tr>
<td align="left"><b>OBSERVACIONES:</b></td>
<td align="left">
<select onchange="var obj= document.getElementById('6');if(this.value=='Otro'){obj.display='otro';}else{obj.display='none';};" >
<?php
$rs = mysql_query("SELECT * FROM obs_call");
for($k=0;$k < mysql_num_rows($rs); $k++){
$obs = mysql_fetch_assoc($rs);
echo '<option value="'.$obs ["obs_id"].'">'.$obs ["descripcion"].'</option>';
}
?>
</select><br>
</td>
</tr>
<tr>
<td><input type="text" id="otro" name="otro" value="" ></td>
</tr>