But the OBSE readme says

1. Copy obse_1_2_416.dll, obse_editor_1_2.dll, obse_steam_loader.dll and the Data folder to your Oblivion directory. This is usually "C:\Program Files\Valve\Steam\SteamApps\common\oblivion".
2. Ensure you have enabled the Steam community in-game, or OBSE will fail to load. Go to Steam > Settings > In-Game and check the box marked "Enable Steam Community In-Game".
3. Launch Oblivion via Steam or by running Oblivion.exe. OBSE will automatically be run along with Oblivion when launched. To disable this, rename or move obse_steam_loader.dll. You do not need to use obse_loader.exe unless you are running the editor.

I wasn't using the OBSE launcher and if I try to, I get an error message telling me not to use it with steam