TIG Postcode validation Magento 2 extension  https://github.com/tig-nl/postcode-magento2 needs accompanying extension to be installed for OneStepCheckout  support https://github.com/tig-nl/postcode-additional-support-checkouts-magento2  

Change-log and release notes:
https://docs.tig.nl/display/TIGSD/Magento+2+Postcode+extensie+release+notes 

Integration details with OneStepCheckout  are described in https://confluence.tig.nl/display/TIGSD/Magento+2+Postcode+extensie+checkout+support