I've experienced the same within oracle user. It turned out that the password of oracle user had expired; I was not able to catch it early as oracle user is not logging into the server directly (only via su - oracle). On another server(server B), I've experienced the same as well but the cause is different. On server B, the cause was, the failed login of oracle user had exceeded on what's allowed.

Here are the steps which helped me and hopefully can help you too:
1. Check the password if expired:
# chage -l oracle

2. If the password is expired, change/refresh it:
# passwd oracle

3. Check again by executing step #1
4. Watch the cron log:
# tail -f /var/log/cron

If password expiration has been sorted out and still getting the same, try the failed login:
1. Reveal the failed login count:
# faillog -u oracle

2. If there are many counts, you better reset it:
# faillog -r oracle

Hope this helps.