Observable.just(fooList)
                .subscribeOn(Schedulers.io())
                .observeOn(Schedulers.io())
                .flatMap(Observable::from)
                .flatMap(foo -> getBarFromServer(foo.getId()))
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Subscriber<Bar>() {
                    @Override
                    public void onCompleted() {
                        doSomeInterfaceWork();
                    }

                    @Override
                    public void onError(Throwable e) {

                    }

                    @Override
                    public void onNext(Bar bar) {
                        SomeClass someClass = new SomeClass;
                        someClass.setShit(bar.getShit());
                        someClass.setFooId(foo.getId()); //!!!!!!!!!!!!!!!! вот тут foo не доступно уже
                    }
                });