1. Stop OMS

    [oracle@hostname ~]$ $OMS_HOME/bin/emctl stop  oms –all

2. Change the IP address:

i)        (IP) vi /etc/sysconfig/network-scripts/ifcfg-eth0

ii)      (GW) vi /etc/sysconfig/network

iii)    (DNS) vi /etc/resolv.conf

iv)    service network restart

3. Start OMS

  [oracle@hostname ~]$ $OMS_HOME/bin/emctl start  oms

After restarting the OMS if you see that all the WebLogic/Middleware targets on the OMS host as down in the console page then follow this procedure derived in Doc ID 1539852.1 :

1. Log in to EM 12c Cloud Control 
2. Navigate to Targets > Middleware >
3. Expand the EMGC_GCDomain target link
4. On "GCDOmain" Right Click
    Refresh WebLogic Domain
    Continue

Once the refresh job completed, click ok and review the status of the WebLogic and Middleware targets of the OMS.

For more info please check Doc ID 1562029.1 and Doc ID 1539852.1 in Oracle Support.