The Buffer Busy Wait alarm occurs when a session cannot access a block because it is in use by another session. The two most common causes are insufficient free lists for a table or insufficient rollback segments.
If the Buffer Busy Wait alarm is current, you should look at the Activity | Buffer Busy Waits Page. This page shows you the actual number and duration of buffer busy waits, as well as a breakdown of buffer busy waits by buffer type.
If the predominant buffer waits are for data blocks, it is likely that you need to create multiple free lists (using the FREELIST clause of the CREATE TABLE statement) for tables that are subject to very heavy concurrent inserts. Waits for data blocks occur when multiple sessions select the same block from a free list, and concurrently try to insert into that block.
If the leading category is for either undo header or undo block, you may need to create additional rollback segments.