Solution : https://service.sap.com/sap/support/notes/432377 (SAP Service marketplace login required)
Key words : 
additional key words psaptemp temporary tablespace initial, oracle data migration assistant, extent odma migration upgrade, allocating/deallocating extents, initial extent, temporary tablespace, migration/upgrade, odma resets, oracle bug #1370923, increased effort
Related Notes : 
       
| 416772 | General tablespace types, temporary tablespace parameters | 
| 359835 | Design of the temporary tablespace in the BW System | 
| 216881 | SMON process slowly deletes temp. segments PSAPTEMP | 
| 183842 | Shutdown (immediate) takes a very long time | 
| 183791 | Locking on temporary tablespace of Oracle |