I have 18.104.22.168 of the OMS installed. I pushed/installed the agent on a Linux host from the OMS. Everything was working just fine. It the host on which the agent was installed showed up in my OMS console and all the databases and other targets showed up just fine. I was going to de-install the 11g Agent from some other hosts and then do the same 12c agent install on those hosts. Unfortunately, I was in the wrong window when I ran the deinstall agent command and deinstalled the 12c agent which I had just installed.
So I figured that I would re-install the agent again from the OMS to that first host. Before doing this, I removed all of the original targets and then the host from the OMS console for that particular machine. I re-ran the agent install and it installed fine. I could not find any way to actually delete the agent from the OMS though.
I re-ran the 12c agent install on the first host which went just fine BUT, now OMS blocks the agent that was re-installed. So I went into the OMS and resynchronized the agent. This unblocked the agent but now there are no discoveries happening. The Agent shows up and running but it does not report the host on which it is running to the OMS nor does it show any of the targets.