I've just submitted v2.3.10 of One-Page Checkout (OPC) for the Zen Cart moderators' review and will post back here when it's available for download.

This release contains changes associated with the following GitHub issues:

#298: Removing duplicate definition from ot_coupon language override.
#302: Correct "Add to Address Book" label, incorrect for=.
#303: Include zc157-added definitions to ot_coupon language override.
#306: Load OPC observer earlier, so it responses to all calls to zen_in_guest_checkout.

Note: This will be the last release supporting Zen Cart versions prior to zc157 (fully patched).