What is the output you get when you do :
emctl start oms
Upload you emctl.log / emctl.msg / emoms.log / emoms.trc files.
Any changes made to the system ?
I have seen similar issue if there is a time difference between the nodes in the cluster in case you are using RAC DB for repository or there is a time difference between the OMS and repository DB host
Please check and verify
Can you provide OEM version? Also upload these files