I have some servers running ESXi 6.5, and I need them to not integrated to any of vCenter servers in the environment, although I need to integrate them directly with the vRealize Operation Manager 6.5 that already existing in the environment.
vRealize Operations Manager leverages vCenter for collecting and as far as i know you cannot directly connect ESXi to vROps.
vRealize Operations 6.5 is also not supported anymore so i suggest to upgrade to 6.6.1 or higher.
Correct, it's not possible to connect ESXi to vROps unless it's managed by vCenter (In which case, you connect vCenter to vROPs)