Is shrink a safe online operation?
As I understand it the resizing of the high water mark requires an exclusive table lock, which in theory might not be a problem as it would be of very short duration. However if it has to wait on other transactions before it gets the lock it will block other transactions.

https://forums.oracle.com/forums/thread.jspa?threadID=2453756

However as the docs point out, you could run a shrink space compact, which does not move the HWM, and run the shrink later on when the system is known to be quiet: http://docs.oracle.com/cd/B28359_01/server.111/b28286/statements_3001.htm#i2192484