The One Step Checkout extension doesn't implement the base Magento totals rendering functionality, so the final points earned and redeemed information doesn't show up.
In versions later than 2.2, Sweet Tooth catalog discounts are sometimes missing or incorrect. A patch for this was released in Sweet Tooth 220.127.116.11. This patch supports Magento 18.104.22.168 and up. If store prices are set to include tax, some problems may persist in versions of Magento prior to 22.214.171.124.
In some older versions of One Step Checkout (v3.1.9), there is also a re-write on the Quote model which poses an extra conflict with Sweet Tooth.
In the new version of One Step Checkout (v4.0.3) and Sweet Tooth 126.96.36.199, the One Step Checkout page is not loaded and it load the default Magento One Page checkout.
To solve, follow these instructions:
1. Open app\code\local\Idev\OneStepCheckout\Block\Summary.php
2. Replace Mage_Checkout_Block_Onepage_Abstract with Mage_Checkout_Block_Cart_Totals on line 10.
3. Open app\design\frontend\default\default\template\onestepcheckout\summary.phtml (in newer versions the file is app\design\frontend\base\default\template\onestepcheckout\summary.phtml) and replace lines 37-66: