You can get error in Oracle that instance is terminated by PMON.
When Oracle Instance terminated by PMON, then go to check Alertlog errors. I have checked alertlog, then I saw following errors.
TNS-00505: Operation timed out nt secondary err code: 110 nt OS err code: 0 Client address: (ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=41361)) 2019-03-27T15:44:35.227680+03:00 Instance Critical Process (pid: 4, ospid: 3321, PSP0) died unexpectedly PMON (ospid: 3317): terminating the instance due to error 490 2019-03-27T15:44:35.341119+03:00 System state dump requested by (instance=1, osid=3317 (PMON)), summary=[abnormal instance termination]. System State dumped to trace file /oracle/product/diag/rdbms/devecidb/deveci/trace/deveci_diag_3337_20190327154435.trc 2019-03-27T15:44:37.035726+03:00 Dumping diagnostic data in directory=[cdmp_20190327154435], requested by (instance=1, osid=3317 (PMON)), summary=[abnormal instance termination]. 2019-03-27T15:44:38.480887+03:00 Instance terminated by PMON, pid = 3317
This error means, there is any problem with Swap partition of Linux. It is not enough space or exist.
To solve this problem, increase swap partition if it exists.
If swap partition does not exist then create it with following link.