I'm trying to launch it from a clean install (using GOG Galaxy) of the game.

I made sure all files associated with the previous install (for both the GOG and Steam Version) are uninstalled and deleted before installing the GOG version.

The game crashes everytime when ever I click play game in the Oblivion launcher.

I've tried verifying / repairing the game file, but it doesn't make a difference.

Anyone got any ideas what the issue could be?

EDIT:

I just tried launching the Steam version too (again after making sure all files associated with the GOG version are uninstalled and deleted) and that does the exact same thing as the GOG version.

So that is both versions that won't work (the Steam version use to work, the GOG version never worked) for some reason.

Does anybody have any ideas what the issues could be?

Possibly a registry issue, caused by an incomplete de-installation of Oblivion.
You can convince yourself by running regedit, searching for oblivion and looking for entries which point to a different folder, probably to the (or a) previous installation folder. If you are not familiar with regedit, I advice against trying this, as you may damage the registry unintentionally.
You could also try to copy your Oblivion folder (or to un- and re-install Oblivion) to the previous installation location and launch it from there. This may or may not work, depending on how messed up your registry already is.
If my guess is correct, then the solution is to uninstall all Oblivion installations you currently have completely, make sure all associated folders have been deleted (including the one(s) underneath documents\my games), and then to clean your registry by deleting everything associated with Oblivion. Again: Don't do this, if you are unfamiliar with the registry and regedit!

I definately made sure all folders containing anything for a current install of Oblivion were deleted after uninstalling the game, for both the GOG and Steam versions. It didn't make a difference.

I also tried installing the Steam version to the exact place it was when that version was working, but it didn't make a difference. Same with the GOG version, though that never worked for me in the first place.

Never heard of regedit, so I'll take your advice and not mess with that.

OK. Here are two links dealing with registry problems for Oblivion:
http://wiki.tesnexus.com/index.php/Oblivion_reinstall_procedure
https://www.nexusmods.com/oblivion/mods/48137?tab=posts

The first one describes a procedure to un- and re-install Oblivion cleanly, which should work, as far as I can say. For cleaning the registry a freeware tool like ccleaner (there are others, and I'm not familiar with this kind of tool) is mentioned in the guide.
The second one offers a mod to manipulate the registry entries for Oblivion. I can't comment in any way on this mod, have no idea what it's doing and if it's applicable to your problem at all.

But before looking further into the registry side of Oblivion, you should perhaps consider other potential problem causes. You run the launcher and get to the launch menu, the one with "Play", "Data Files", etc. What exactly happens when you click "Play"? Error message? If yes, which one? CTD (immediate crash to the desktop)? Does the PC hang? If the PC hangs, can you get out by using ALT-TAB, or by using the task manager, or do you have to switch off your PC? Anything else?
There are so many thing you could attempt, like playing with the game options or with the oblivion.ini (the simplest method is to delete it, the one in "mygames", and to restart Oblivion). There could also be a CODEC issue (for example preventing the intro videos from being played), or the problem Hastur mentioned. It might also be worth trying if installing UOP (the Unofficial Oblivion Patch) helps, or disabling the official addons in Data Files (should you have installed them).

Unfortunately it's like searching for a needle in a haystack. My guess that there might be a registry problem underneath was based entirely on the fact that the Steam version worked in the past, but both the GOG and the Steam version don't work now. This phenomenon can only be explained by a registry issue or by significant changes to your PC, be it new hardware (for example a new graphics card) or new software interfering with Oblivion.
Have you tried installing the classic (GOG) version of Oblivion? Not that I have any indication that Galaxy causes this kind of problem, but it adds a layer of complexity with the potential that more things can go wrong.

Richad, so no special characters are used.

when I click play in the GOG version, Galaxy syncs my save data as if I have exited the game, but the game never actually starts. It doesn't appear in task manager either.

For the Steam version it's the same, nothing happens when I click start, doesn't appear in the task manager either.

My PC doesn't hang or slow down while any of this is happening either. I can use and open other programs without any issues while this is happening.

The classic GOG version behaves the same as the Galaxy version.

I have changed graphics cards, but that was when the Steam version was still working.

So the crash happens before the game task appears in the task manager. I'm not knowledgable enough to interpret this information adequately. A registry problem is still not unlikely in my mind.
Things to try (no guarantee that it helps):
- Compatibility to Win98.
- Is the DPI-Scaling box in the compatibiity window ticked? If not, tick it.
- Delete oblivion.ini in my games
- Run oblivion.exe directly, instead of the launcher
- Oblivion.ini: Is the parameter bUseMyGamesDirectory set to 1? If not, set it to 1 (it makes sure that the oblivion.ini in my games is really used, otherwise Oblivion uses the obllivion.ini in the main folder, if present)
- Run Oblivion in a window (in oblivion.ini change value of bFull Screen parameter to 0)
- Disable all intro movies (in oblivion.ini remove all values of the 2 Intro lines, which should read
SMainMenuMovieIntro=
SIntroSequence=
Before you modify the oblivion.ini, create backup. Alternatively delete oblivion.ini in order to reload the default ini.)
- Disable UAC and/or DEP for Oblivion.
Can't think of anything else right now.
Good luck!

Have you redirected your Documents folder to One Drive?
The game may not have permission to access the files.

OK I've figured out why it wasn't working.

For some reason, despite me not having to do this for literally any other install of Oblivion I've ever had, be it the disc version, Steam version or GOG version, this install wouldn't work unless I set the game to run as an administrator.

If anyone could tell me why this is it would be greatly appreciated.

My default GOG installation runs without requiring admin rights - no prompt, admin box in properties not ticked.

likely it involves some kind of security feature of windows so Microsoft will never tell you the reason.

Sorry for the necro, but for anyone needing help with this try running it in admin mode and also replace the contents of Oblivion.ini with the text in Oblivion_default.ini in the installation folder. That's what made it work for me.

My guess is that there is some form of protection with some extra scripts by gog, but it's weird that it would cause the steam version to not work as well.

Edit: Also remove the Oblivion_default.ini file from the game folder, and run the launcher first, after running it in admin maybe try running it without admin mode.

Edit2: Try running it twice, Like it opens and stuff on the first try and check the task manager for an oblivion.exe program then run it again, I don't know why it is like this, but it just works like this

I don't know if it's the same problem, but I can't get the launcher to work at all. Going straight into the game I have no problem, but the launcher always says memory can not be read. I've also had multiple copies installed in the past and had the same problem when I bought the steam version before too.

If I delete the documents stuff, it will launch clean but crash after trying to detect my hardware. I feel like if I can just get it to skip that step I'd be fine. Also, when I try to launch using OBSE, it says it can't detect the version.

(BTW this was from a clean install from GOG onto an external drive)

could be a driver or hardware problem, did you try updating your drivers