In the docs for OPC you'll find the fix to help with the MooTools use.
The plugin's design also utilizes jQuery as its AJAX interface to the various updates required. There are some Zen Cart plugins that use the MooTools interface that will need a little extra "care and feeding" for compatibility with One-Page Checkout. OPC's v1.0.8 release provides the changes that enable interoperation with MooTools, but some plugin-specific changes might be needed in addition — specifically:
  1. Order Delivery Date. You'll need to copy that plugin's jscript*.* files to the /includes/modules/pages/checkout_one directory and modify the file jscript_a-mootools.js, adding the highlighted line to the top of that module:
    //MooTools, , My Object Oriented (JavaScript) Tools. Copyright (c) 2006-2009 Valerio Proietti, , MIT Style License.

jQuery.noConflict();