http://jsfiddle.net/alexnode/y4y4A/8/
#bgmbutton1{position: fixed;
left: 2%;
top: 5%;
}
#button1 {width: 25%;}
#obutton1 {width: 25%;position: absolute;top:0;left:0;}
#otrigger1 {background:rgba(0,0,0,0); height:100%; width:100%; position: absolute; left: 0; top: 0; cursor: pointer; z-index:5000;}
You should add background:rgba(0,0,0,0); to create an invisible transparent overlay trigger. check the fiddle above.