I've just submitted v2.4.0 of One-Page Checkout for the Zen Cart moderators' review; I'll post back here when it's available for download.

Note: This version drops support for Zen Cart versions prior to 1.5.7a. If your site is using an older Zen Cart version, please continue to use OPC v2.3.12.

This release contains changes associated with the following GitHub issues:

#304: Drop support for Zen Cart versions prior to zc157a.
#312: Additional AJAX messages to distinguish various timeout scenarios.
#313: Improve performance on admin's orders' listing display.
#315: Correct address handling when "Enable Shipping=Billing" is set to 'false'.
#316: Render OPC overlay when an address (either billing or shipping) is changed via dropdown of existing addresses.
#317: Enable interoperation with the square_webPay payment method.
#318: Add a breadcrumb on the order_status page.
#319: Use zen_cfg_read_only for OPC's "Version" value.
#320: Correct PHP Fatal error received when an issue requiring a customer message is detected by OPC's observer.
#321: Add zone_code to data supplied by the NOTIFY_OPC_INIT_ADDRESS_FROM_DB notification.
#322: Correct number of parameters supplied to zen_prepare_country_zones_pull_down.
#324: (Partial) Additional information gathered for "Invalid request, tempAddressValues not set." error log.