This is the way to do this, you can also set the stroke. Be careful, if you're trying to take the contents code from [this site][1], remember to put the "u" character inside the string, like this: '\e059' -> '\ue059', otherwise it will not work.

    var styleIcon = new ol.style.Style({
    text: new ol.style.Text({
        text: '\ue059',
        font : 'normal 18px "Glyphicons Halflings"',
        stroke: new ol.style.Stroke({ color: 'black', width: 2 })
        fill: new ol.style.Fill({color: 'yellow'})
        textBaseline: 'Bottom',
        scale: 1              
    })

Here is a working [jsfiddle][2] 


  [1]: https://glyphicons.bootstrapcheatsheets.com/#
  [2]: http://jsfiddle.net/koba1/rujbeovk/1/