foo.js

o.toObservable = function () {
  var o = this;
  return Rx.Observable.create(function (obs) {
    function handler (e) {
      obs.onNext(e);
    }

    Object.observe(o, handler);

    return function () {
      Object.unobserve(o, handler);
    }
  }
}

o.toObservable().select(function (x) { return x.newValue + x.oldValue; });