oracleasm createdisk command 

[root@oracle12cdb1 ~]# oracleasm createdisk CRS1 /dev/sdb1

Writing disk header: done

Instantiating disk: done

[root@oracle12cdb1 ~]# oracleasm createdisk DATA1 /dev/sdb2

Writing disk header: done

Instantiating disk: done

[root@oracle12cdb1 ~]# oracleasm createdisk FRA1 /dev/sdb3

Writing disk header: done

Instantiating disk: done

[root@oracle12cdb1 ~]# oracleasm listdisks

CRS1

DATA1

FRA1