Drupal error. Drupal safe mode possible?
Try deleting the row for the og module in the system table. Then run a query for each database table beginning with cache_:

TRUNCATE TABLE cache_*


This should make Drupal forget anything it knew about organic groups (this is how to clear the cache manually).

I'd suggest keeping your modules in the standard location which is /sites/all/modules rather than /sites/default. I don't know if thats part of the problem here but it might be.