xxx_objects.data_object_id differs from object_id after
 truncate table (or alter table truncate partition), unless table/partition is empty
 alter table move (or alter table move partition)
 alter index rebuild (or alter index rebuild partition)
 alter table exchange partition
Alter table move even without TABLESPACE clause (or specifying the same tablespace) physically moves the table, based on dba_extents.file_id and block_id. But truncate table doesn't move the table. Think of data_object_id as an ID for the segment. If xxx_objects.data_object_id is null, it must be an object not associated with a physical segment, such as view, sequence, domain index, partitioned table/index whose segments are in the individual partitions.

* How much does data_object_id increment?

Except in case of partition-table exchange, data_object_id is brought up to

select max(data_object_id)+1 from dba_objects

If you just create a new table, it will be assigned an object_id and data_object_id of

select greatest(max(object_id), max(data_object_id)) from dba_objects

If the table has one index, truncate will increment data_object_id by 2 because its index takes the number one below it. If there's no index, truncate should increment data_object_id by 1 only.

Exchange of a partition with a table swaps their data_object_id's. So don't assume data_object_id's always go up.

If the table or its partition is already empty, truncating it does not increment data_object_id. (Rebuilding an empty index or its partition still increments data_object_id.)

* When to use which ID?

You use data_object_id in dbms_rowid, v$bh and x$bh, but object_id in most other cases, such as v$locked_object. V$segstat, v$segment_statistics and v$logmnr_contents have both.