There are two types of Masters in Oracle RAC, one is Mater node at Oracle Clusterware level and other is Master node for specific resource or block or object.

At the Clusterware level , the node that store OCR backups is the master node

Eg :

Here rac1 is the master node :

[root@rac1 ~]# ocrconfig -showbackup

rac1 2012/11/15 16:46:15 /u01/app/11.2.0/grid/cdata/rac-cluster/backup00.ocr


Block Resource Master :
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In RAC, every data block is mastered by an instance. Mastering a block simply means that master instance keeps track of the state of the block until the next reconfiguration event (due to instance restart or otherwise).

Hash to the master

These data blocks are mastered in block ranges. For example, range of blocks starting from file 10, block 1 through block 128 may be mastered by instance 1, blocks from file 10, block 129 through 256 are mastered by instance 2 etc.

The block level masters are used by Cache fusion while transferring the block. Any node can become the master node of a particular block and you can also see which node acting as master in V$GES_RESOURCE table (MASTER_NODE column)

You can manually remaster an object with oradebug command:
SQL> oradebug lkdebug -m pkey "object_id"