VAT validation

One Step Checkout module does not add any tax-related functionality, all the general information on Magento VAT functionality applies. Magento has two different vat fields and you can use both also in One Step Checkout .

Magento 1 has built in VAT validation, please see Magento manual how to activate it.

Please test in standard checkout and once it works as needed, try in One Step Checkout and it should work.