The news that Peter Orszag will leave the OMB in July is not surprising; he was set to leave in April but was persuaded to stay on a bit longer. Orszag will be remembered chiefly for two things: his key role in designing the stimulus and the new health care law and his odd status as the Justin Bieber of the executive branch, the wonk whose personal life was splashed all over the tabloids. Orszag's true legacy won't be established for years, of course, not until his ideas about how to "bend the cost curve" of health care expenditures--many of which were crammed into the new law--have been tested in the real world.
It probably won't get much attention, but Orszag deserves particular credit for OMB's performance in the early days of the administration. It's a somewhat obscure, "how government works" point, but OMB is often most valuable to the White House during transition years between administrations because it has a large staff of expert bureaucrats that doesn't turn over the way that, for instance, the Treasury Department's does (and 18 months into the administration, many Treasury posts are still vacant). This was particularly important when Obama took office, given the scale of the economic crisis and the imperative to pass a major stimulus. Orszag's leadership of OMB during this period is an underappreciated accomplishment.