You can control product visibility from the product page in WooCommerce.
To HIDE a product from your online store so that it is only available in-store via Oliver POS, go to the Edit Product page in WooCommerce. Under the Publish section, set Catalog visibility to Hidden and click OK:
If you don't have any other changes to make to your in-store-only product, click Update to save the new Catalog visibility setting.
You can verify that the product has been hidden by doing a quick search in your webshop:
Meanwhile, the product is still available for use in-store via Oliver POS: