The error says "require(\bootstrap.php)", which would indicate the APPPATH contant contains "\", which is absolutely wrong.
It is set on line 35 of the same "oil" file, and I assume your fuel/app folder is either not in the default location (which means you have to adjust the paths), or __DIR__ doesn't work (which indicates you're not using PHP 5.3.3+).
Oh I understand why It didn't work. The Oil script is configured to work by default at the same directory level of Fuel package. We can see it by the constant statement
I moved my Fuel package in a phpapps package and the Oil script was no more at the same level of Fuel package. It is normal I got an error. I moved Oil into phpapps package. Now It should be fine.