Use 'filter' function to define how to filter items.
<select class="easyui-combobox" style="width:200px;" data-options="
filter: function(q,row){
return row.text.toLowerCase().indexOf(q.toLowerCase())==0;
}
">
<option value="NC">North Carolina</option>
<option value="ND">North Dakota</option>
<option value="OH" selected>Ohio</option>
<option value="OK">Oklahoma</option>
<option value="OR">Oregon</option>
<option value="PA">Pennsylvania</option>
<option value="RI">Rhode Island</option>
</select>