There is a separate page to administer product types, complete with a "Create new product type" button.
Using django-admin is not a good solution, you may be able to add product types and products through it, but you'll be missing out on any dashboard hooks into the normal process.
A look at the source code shows that whilst you may be able to add a product without a type (the FK is nullable), you may then experience other problems down the line as oscar expects only child products to have a null product_class.
#: None for child products, they inherit their parent's product class
product_class = models.ForeignKey(
'catalogue.ProductClass', null=True, on_delete=models.PROTECT,
verbose_name=_('Product Type'), related_name="products",
help_text=_("Choose what type of product this is"))
Definitely best to try to work with the system rather than around.