OPM began as the Civil Service Commission many years ago when there was a need to show the American public--unquestionably--that the U.S. Government hires and manages employees based on merit and not because of a person's connections. One can argue whether anyone has ever been convinced of that theory, but I digress.
The "Management" parts of OMB and OPM overlap at some points and even where they don't overlap there is no longer a pressing need to staff up an entirely separate operation just to prove a public point. We may do well to merge the two organizations, one that has a "Management" side and a "Budget" side within the singly-headed organization. Money would be saved by having one rather than two political appointees, and by the de facto reduction in the number of subordinate political appointees. Today (and always in the past) OPM has appointees who serve similar functions to those in OMB.
The audit functions that both organizations perform can also be more efficiently done if managed singly rather than doubly.
Personnel policy can be established just as easily at OMB as it is in OPM... just bring the staff over (and reduce some of the extraneous SES number in the process).
In addition, a government-wide savings can be realized by bringing back in to the appropriations process the reimburseable "services" OPM currently offers.
Government agencies should do things that are inherently governmental. Security background checks and merit-system recruitment and examining for other Federal agencies are all so important that our government's agencies should not have to shop around for the best deal. Wait, that's not completely the case right now; when it comes to security checks, agencies have to shop at the "company store" and pay for services that are required of them (which is ludicrous). And when it comes to making sure agencies actually follow the merit system principles established by the Civil Service Commission all those years ago, how is that NOT inherently governmental? HR functions were decentralized back when automation and fantastic information technology was merely a dream for most people. With today's capabilities, we can improve the quality and timeliness of the hiring process, and secure a better chance that merit system principles may actually be followed more often than they are today.
Merge OMB and OPM, authorize appropriation of services that agencies now must pay for out of their shrinking budgets, and help come closer to realizing the goals of Theodore Roosevelt for a personnel system based on merit.