Hello,
Most of the time that happens because you have your game installed into the program file folder(UAC).
Install your game anywhere else, like into a folder on the C drive you name games. Windows protects changes on the programs files folders.

Maybe it's Window's UAC magic. A short while ago someone here in this forum had their personal folder write-protected, or relevant parts of it.

- Are you using a savegame manager?
- Do you have two different Oblivion installations, or have you uninstalled Oblivion previously?
- Are you using something like Galaxy, which might have a different way of managing savegames, or are you using cloud saving?

Every yes to one of these questions might cause the behaviour you're describing (just to mention a few possible explanations). Without further details about your Oblivion installation and configuration it's impossible to help.

installed Oblivion on D:\Gog Games\Oblivion
other than the standard save game which automatically installs to the default for saves, no, nor do I know what a "Save manager" is. please link where I can download it.
I am using Galaxy but I've found a way to go in and untick the "read only attribute" in the folder of the saves which is
C:\C:\Users\matt\Documents\My Games\Oblivion I had to untick it in the Oblivion "directory" there and also the "Saves" directory. it seems to re tick it when I reboot the PC. also the game insists on putting my saves there I have no choice in the matter on that, only where I install the game!
I have managed to get the saves to work (some of the time) by doing the above as well as running the game as Administrator and disabling "cloud" saves as I don't like that anyway.
for some reason it only saves part of the time with my first "Imperial" character but so far, anyway, with my new Khjiit it saves every time, upto save 18(here's hoping) starting a new game seems to have maybe cleared the problem as I did the above steps. I noticed it gets flakey if I don't "check" the game when I first play and save immediately and then "load" the save to see it there. If I do not save after starting the game and play for about 30 minutes the saves do not work or are flakey. the autosaves only work some of the time not all the time! WTF kind of laws of physics is that! this could be as when I "untick" the save folder it reticks on reboot. that's windows 10 for you. wants to be a mother hen!

Sounds like your problem is Galaxy-related. Try the non-Galaxy version instead (what's the advantage of Galaxy for playing a game like Oblivion anyway, when you're not interested in cloud saving? - well, it's your choice).
For savegame managers search this forum. There are several different ones available, with different functionality, some one-purpose tools, some components of larger utilities. You can also do it manually, by creating a folder for every character you play and moving the respective save games from the standard folder to the character's folder at the end of the session with that character, and from there back to the standard folder before you start a new session. This lets you play different characters in parallel, and is essentially what savegame managers do, too.

UAC is horrible with older games from my experience. The easy way to fix it is to go into control panel and set UAC to lowest, but only do this if your PC is safe from random installs by random people.

Did you get it fixed, I'm having same problem

of late I've found that since Spring Update 2021, disabling ransomware protection's "controlled folder access" fixes most of these save write problems. this is also true in many other games like "Orcs and Men" and "Spellforce 3" and all the "Batman Arkham" series. even if they are new and have no relation to Elder Scrolls lots of games do not access the disk at all if they run through Defender. regardless of which drive you place your game folder in. If your achievements in (lots of) games some how one day fail to upload it is likely ransomware protection is enabled. Until windows screws up again with the "Fall update 2022" this seems to work well.