I'm not on O2 myself so couldn't answer for that but can confirm Flip 5 should be able to access 5g standalone app be it it is very much in its infancy and only available in a very small number of locations in certain cities.
I am not employed by Samsung nor do I receive any remuneration as a Samsung Members Star.
5G Standalone (SA) is a type of 5G network architecture that operates independently, without relying on existing 4G networks. 5G SA networks are designed to support advanced use cases, such as: Ultra-reliable low latency communications (uRLLC), Massive machine type communication (mMTC), Enhanced Mobile Broadband (emBB), and Critical IoT (Internet of Things).