var l = map.layers[1];
for(var i = 0; i < l.features.length; i++){
    var f = l.features[i];
    f.style = {display : 'none' };
    l.drawFeature(f);
}

alert('features hidden');
for(var i = 0; i < l.features.length; i++){
    var f = l.features[i];
    delete f.style
    l.drawFeature(f);
}