By default Experious PostcodeNL extension https://github.com/experius/Magento-2-Module-Experius-Postcode-NL/ errors out on conflicting jsLayout (
https://github.com/experius/Magento-2-Module-Experius-Postcode-NL/issues/47 ). All uiComponents in this extension are added with layoutProcessor and not with layout xml to get it to work with OneStepCheckout you would need to:

  1. add a field-set component experius_postcode_fieldset to Stores > Configuration > Sales > OneStepCheckout > Shipping address fields and settings and drag before street field

  2. open up following file inside ["experius/module-postcode" installation path]/Block/Checkout/LayoutProcessor.php and in method getPostcodeFields comment out all the sortOrder nodes in their uiComponents inside a field-set

    //'sortOrder' => (string)($this->getStartSortOrder() + 50),