Hola julianserna.
Prueba esto:
Código PHP:
<html>
<head>
<script language="JavaScript">
function activaOtro(){
with (document.frm){
otro.disabled = (sexo[sexo.selectedIndex].value != "O");
if (!otro.disabled)
otro.focus();
}
}
</script>
</head>
<body>
<form name="frm">
Sexo:
<select name="sexo" onchange="activaOtro()">
<option> </option>
<option value="M">Masculino</option>
<option value="F">Femenino</option>
<option value="O">Otro</option>
</select>
<input type="text" name="otro" disabled>
</form>
</body>
</html>
Saludos.