How to call a js method in my template with RequireJS in Magento2?

You can't do it in template, and you can't access directly to a private method. You can do it in a requireJS module through the mixins configuration.

See the docs.

In your case you could do something like his:

var config = {
    'config':{
        'mixins': {
            'priceOptions': {
                'Vendor_Module/script' : true
            }
        }
    }
}; 

And in your script.js

define(['jquery'], function ($) {

    return function (widget) {

       $.widget('mage.priceOptions', widget, {

           yourMethod: function () {
                 this._onOptionChanged
           }

       });
       return $.mage.priceOptions;
   }
});