Note that the OSS and IOSS rules come into force for your company not immediately after the registration but only at the start of the next quarter after the application. One more rule is that you cannot combine OSS or IOSS with the usual VAT reporting. So if you've registered for OSS or IOSS, you must use only these schemes for all your sales within the EU. Notice, it is not the case of the country of the company's incorporation: using OSS or IOSS schemes,
you shouldn't forget about the VAT requirements in your country — remember that these programs work only for non-domestic VAT liabilities. You can find more information about OSS and IOSS and explore a bunch of tables
here.