Found couple of simmilar bug reports on metalink (ORA-600 [711][...]) on open database operation, most of them on the older releases, though. But they are all related to corrupted index tablespaces. here are two example bug numbers from metalink: 856652, 1341922.

Is your database trying to perform instance recovery during ALTER DATABASE OPEN? Ie, do you see any messages "Beginning crash recovery.." in your alert file after "alter database open"?

If so, you might want to run dbverify over your database datafiles to see if any of them is corrupted. The hits from metalink make me belive that you'll find some of the files that hold database indexes to be corrupted....