Can I also link my savings and investment accounts to OkiOki?

No, this isn’t possible (yet). Information from savings and investment accounts falls outside of the PSD2 legislation and is therefore not shared by the banks. 

In the next PSD2 update, called PSD3, which will probably be released in 2030, savings accounts, investment accounts and insurance products will probably be available.

However, at the moment, you can manage your credit card expenses with OkiOki already.