The Netherlands Bureau for Economic Policy Analysis, which is similar in many ways to the OPBO, has an interesting FAQ about independence:
CPB is a research institute that is independent with respect to content, but at the same time is formally part of the central government. This ambiguous position, which we share with the other planning offices, often raises questions.I think the last sentence is key. It is up to our politicians to make the decisions, but they can make better decisions if they have solid numbers on which to base their decisions. And that is what an effective OPBO can do.
CPB does not find this position to be constraining. Succeeding Ministers of Economic Affairs have respected and, if necessary, defended CPB's independence, even at times when they could not agree with the conclusions drawn by CPB. Most politicians accept and appreciate CPB's independence. This enables us to work for the Cabinet and for the opposition at the same time. After all, politics takes decisions regarding the chosen policy, whereas CPB - on demand or on its own initiative- seeks to clarify the economic effects of specific policy proposals.