Paying with OK is as secure as paying via PIN, iDEAL or banking apps.
First, the OK app is only useful on your mobile phone, so the access code or fingerprint on your smartphone provides the first security.
Secondly, your OK app is linked to your data that is securely stored in your OK account. Finally, you must confirm OK requests (transactions) with your 5-digit OK code.
At last, only you decide what information you provide, for example, to a store or webshop.