solved
readme instructions work for steam ,but are not well optimized
my version is steam goty deluxe
and you dont have to overwrite your nehrim installation to your oblivion contents in in steam apps resulting ~8,8GB size
A
intall nehriim anywhere you like it will find the required data from oblivion automatically ~5,6 GB
B
copy to nehrim directory OblivionLauncher.exe & Oblivion.exe from oblivion in steam
C
rename your oblivion folder in steam to something else eg oblivion.bak
D
put nehrim into steam and rename nehrim folder to oblivion
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you can play both via steam, as long as the one that you'd like to play is
renamed to oblivion and the other-the one you dont wish to use- is renamed to something else
you use only the oblivion launcher to start both games (nehrim launcher doesnt work with steam.exe)
only major incovience is that the first time after each name swap
you should recheck the corresponding data files
because of the shared appdata file
but I adapted, I found OblivionModManager and the change between data files is pretty quick now