Only dedicated servers have static IP addresses, however these can be changed in the event of architecture migration or disaster recovery.

Shared servers are regularly consolidated, hence instances hosted on those servers may be moved from one server to another at any time.

Since it is subject to change, you should not use A records or otherwise rely on the IP address of a Server being constant. 

All traffic and whitelisting should use the domain.

If you are require a static IP address you can host Odoo On Premise, including a cloud provider that gives you static IP address capabilities.

If you want to get server IP address, you should login odoo server with SSH client and run ifconfig , IP address flow eth0:inet.

Hope it's useful to you