Is the Order Attributes compatible with the One Step Checkout extension?
The extensions have been tested and confirmed to work smoothly together. Order Attributes is included in the Premium package of One Step Checkout. There's an option to test both modules together on the demo version of One Step Checkout Premium.
It is possible to add Order Attributes of various types to be displayed on the One Step Checkout and define the position for the attribute to appear on the checkout page. Please see an example of an order attribute on the checkout configured as follows:
Here is the frontend result:
Additionally, it is possible to adjust the Order Attribute sort order using the Manage Checkout Fields functionality provided by the One Step Checkout extension (Amasty → One Step Checkout → Manage Checkout Fields):