Is it necessary to apply for whitelisting of IP addresses before using OPA?

It is not necessary in the development environment, as there are no IP address restrictions.
If you use the production environment, you need to whitelist the IP address on PayPay side.
Please specify the IP address when you conduct an official application on PayPay for Developers. The procedures for official application are as provided below:
https://paypay.ne.jp/developers-faq/paypay3/post-59/