Having same issue on OEL 6.3 and OHS is 18.104.22.168
In case I will stuck on same issue again (and for all the ppl around the internet): this is because of misconfigured or unconfigured /etc/hosts.
Add the IP addresses there and everything will be OK.
Thanks drookie, you solved my problem.
I was seeying the same error as the OP, i saw this in the .out file log, not in the .log :
3opmnctl startall: starting opmn and all managed processes...
In line 6 of ********/config/OPMN/opmn/opmn.xml:
LSX-00291: value "-1" is less than minimum "1" (inclusive)
LSX-00291: value "0" is less than minimum "1" (inclusive)
LSX-00213: only 0 occurrences of particle "sequence", minimum is 1
[2013-09-09T15:31:36+00:00] [opmn] [ERROR:1]  [internal] XML schema validation failed: error 213.
opmnctl startall: failed.
oracle.as.config.ProvisionException: Unable to start opmn
I had the same 3 hostname (with 3 differents IP) into my /etc/hosts files, i removed the two (witch was invalid), then restart the install again (but i had to delete the instance in the wls server first), and it worked !