You can do something like
comboViewer.setContentProvider(new ObservableListContentProvider());
comboViewer.setLabelProvider(labelProvider);
// input must be a List
comboViewer.setInput(input);
IViewerObservableValue swtObs = ViewersObservables.observeSingleSelection(comboViewer);
Now create a model-ovservable and bind it with swtObs