Edit in JSFiddle

var isFocus,
  inputs = document.getElementsByTagName('INPUT');

for (var i = (inputs.length - 1); i >= 0; i--) {
  inputs[i].onfocus = function() {
    isFocus = this;
  };
}

document.getElementsByTagName('BUTTON')[0].onclick = function() {
  if (isFocus) isFocus.value = 'OK';
};

<input type="text" id="o1">
<input type="text" id="o2">
<input type="text" id="o3">
<button>Click</button>