since part OC has been issued builder can force you to take possession of your flat
2) you can take possession as there is partial OC issued .Where the Chief Executive Officer on the certificate of the technical personnel is satisfied that the temporary occupation of a building or any portion thereof before its completion does not adversely affect public welfare, may permit temporary occupation of such building for a period as he deems fit.
3) It is valid until the entire project is completed and the authorities issue the Final OC for the entire project