Does Oracle write to data files in begin/hot backup mode?

Oracle will stop updating file headers, but will continue to write data to the database files even if a tablespace is in backup mode.

In backup mode, Oracle will write out complete changed blocks to the redo log files. Normally only deltas (changes) are logged to the redo logs. This is done to enable reconstruction of a block if only half of it was backed up (split blocks). Because of this, one should notice increased log activity and archiving during on-line backups.