Honestly I don't see why OOS11 gets love over OOS12. It runs slower out of the box because of TPD throttling on most common apps, 5G is much more finicky, battery life is worse, etc.For what it's worth, OOS 11 did not give me the performance on the 5G Network that OOS 12 is giving me now.
OOS 11 I simply edited the policy restrictions in order to access the 5G cell towers and the download speeds were 50% slower than a comparable 5G capable handset (iPhone 13 pro max, Motorola Razr 5G, One+ 8T 5G). Even after unlocking the bootloader, rooting and reapplying the modified policy restriction.xml the One+ 9 Pro on the OOS 11 had poor 5G download speed.
OOS 12 with unlocked bootloader, rooted, and using 5G switch (playstore app) the 5G download speeds improved greatly. I would say they are 50% faster than same handset on OOS 11. I cannot access engineering mode and therefore modify the policy restriction.xml file like on OOS 11, but given the faster download speeds I'll take it for now. For what it's worth, 5G switch would not work on OOS 11.
Another great option that OOS 12 has is a separate volume level for Ringtone and Text Tone. I NEED to be able to hear my text tone alert (which is usually a very short burst) all the while keeping the volume of the ring tone down so that I can take my time to analyze who is calling without disturbing those around me. OOS 11 combines ring tone and text tone under one forced volume setting, which blows imho.
I personally don't find the interfaces of the two much different, and cannot say that I have noticed any real speed differences or battery consumption pros and cons. I do like the graphical design of OOS 11 over OOS 12, but that can be improved by using different Launchers if that is your thing.
I don't have many complaints about OOS12 honestly. It's not perfect but it's not anywhere near as bad as everyone tries to make it out to be. I flashed every custom ROM that was available for my 9 early last month and I also tried COS C.62 and I went right back to C.48 because of issues ranging from weaker signal on custom ROMs (confirmed by actual signal readings, which on average, were 10dB lower on a custom ROM than OOS12) to UI stutter and a bunch of minor annoyances in-between like the finger print sensor overlay being in the wrong place.
Rooted C.48 global has so far been the best experience for this phone for me and while I haven't tried Nameless that everyone seems to love as it wasn't an option at the time, I don't think I'm going to go through the hassle of going back to a custom ROM.