After removing an older BKChem version installed via Ubuntu 10.04 into a Python2.6 environment I cloned the oasa, bkchem, bkchem-plugins, and piddle from Gitorious:
git://gitorious.org/oasa/oasa.git
git://gitorious.org/bkchem/bkchem.git
git://gitorious.org/bkchem/bkchem-plugins.git
git://gitorious.org/bkchem/piddle.git

In the oasa directory I executed, as per the usual:
python setup.py install --prefix=/usr/local/

No errors were detected and import oasa worked fine.

In the bkchem directory I executed, as per the usual:
python setup.py install --prefix=/usr/local/

This gave the following error:
package init file 'bkchem/oasa/__init__.py' not found (or not a regular file)
error: package directory 'bkchem/oasa/oasa' does not exist

The bkchem setup.py file packages = line contains bkchem/oasa/oasa even though this directory does not exist in the git repo (although the oasa directory does, but is blank).

Upon removal of the 'bkchem/oasa/oasa' and 'bkchem/oasa/oasa/graph' packages no errors were encountered in setup.py

To make the setup.py more general it could either check for the existence of the oasa directory in the bkchem package or see if oasa imports and is a compatible version.