You could sort them in-place:

<ol class="items" data-bind="foreach: getDisplayedItems().sort((a, b) => ko.unwrap(a.label) < ko.unwrap(b.label) ? -1 : 1)">
    <li class="item">
        <a class="simple" data-bind="html: label, attr: {href:url}, visible: count >= 1, css: { active: is_selected }"></a>
        <span class="count" data-bind="text: count"></span>
    </li>
</ol>

but I think that's ugly, and it's not re-usable. It's better to make a new observable array that contains the items in the right order:

function App {
    // the observable you already have
    this.getDisplayedItems = ko.observableArray([...]);

    // a computed observable based on that
    this.getDisplayedItemsAlphabetical = ko.pureComputed(() => this.getDisplayedItems.sorted((a, b) => ko.unwrap(a.label) < ko.unwrap(b.label) ? -1 : 1));
}

and

<ol class="items" data-bind="foreach: getDisplayedItemsAlphabetical">