The easiest solution is to create a proper constructor function for your object:
var MyObject = function(){
this.foo = ko.observable("foo");
this.bar = ko.observable("barr");
this.foobar = ko.computed(function(){
return this.foo() + this.bar();
}, this);
}
And use the constructor function to create your object instead of the object literal:
var ViewModel = function () {
var self = this;
self.obFoo = ko.observable(new MyObject());
}
Demo JSFiddle.