Perhaps others wouldn't consider this a bug, but it was definitely surprising to me. When switching between optimisation levels -OO (optimise and strip docstrings) and -O (just optimise), you will find the docstrings are still stripped. E.g.

$ ls
hello.py world.py
$ cat hello.py
import world
$ cat world.py
def foo():
    '''I'm a docstring'''
    pass

import sys
print 'optimization: %d' % sys.flags.optimize
if sys.flags.optimize < 2:
    print foo.__doc__
else:
    print 'optimisation is enabled; no docstrings'
$ python -OO hello.py
optimization: 2
optimisation is enabled; no docstrings
$ python -O hello.py
optimization: 1
None
$ rm world.pyo
$ python -O hello.py
optimization: 1
I'm a docstring

Is this behaviour intentional?

Not really intentional, it's just a limitation of the current implementation (where the same .pyo files are used for both -O and -OO). Already reported as issue1538778 (which was closed as won't fix).

Ah thanks, Antoine. Sorry, failed to find that issue in my prior searching.

It's something that could potentially be changed in the future, but no one has prioritized it enough to worry about it. Typically this opens a pandora's box of wanting to generalize it so it supports custom user optimizations as well and then people lose motivation to push it through to the end. =)