targeting browsers on mac systems

[hablablow]

Hi,

I use this script below to show / hide specific layer content for Opera browsers. Unfortunately it does the correct job only on windows but not on mac.
Any idea ? Add lines to target mac Os i imagine ?
Any help is greatly appreciated...
Thanks in advance...
Guillaume

THE SCRIPT:

function killOpera() {
var op = navigator.userAgent.indexOf('Opera') != -1;

if (op) {
document.getElementById('topSword').style.display = 'none';
document.getElementById('topSwordOpera').style.display = 'block';

}
}

onload = killOpera;