var words= {a: 1, b: 2, c: 3};
<select ng-options="k as v for (k, v) in words">
<option value="">Choose word</option>
</select>
var words = [ {key: 'a', value: 1}, {key: 'b', value: 2}, {key: 'c', value: 3} ];
ng-options="word as word.key for word in words track by word.value"