Difference between OPM and PMO

Physicians & OPM Alumni general-discussion-3
1

Hi,

I believe that this topic has been discussed in past. But I am still not clear about the difference between OPM and PMO… I have read the definitions given in PMBOK and Rita, but still unable to visualize the difference.
Also one more question… Does Portfolio management comes under PMO or is it different? If Project Management Office and Portfolio Management office is one and same, then it feels and OPM and Portfolio Managment Office (PMO) do the similar function… Or can OPM be visualized as a top most layer/umbrella over PMO?

Any help will be apprecited.

I didn’t find the right solution from the Internet.

References:
http://forum.izenbridge.com/viewtopic.php?t=6047
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Thank you.