Let’s look at each of these in a little more detail.
Alignment of Management Expectations:
The importance of alignment between expectations and capabilities cannot be overstated. A one person PMO is not going to rescue all of a company’s troubled projects, align and balance its project portfolio, create and deploy a product development life cycle and train/coach/mentor the project managers.
Similarly, a PMO with all junior staff will be less effective than a PMO staffed with experienced personnel. What can be accomplished is determined by both the number of staff and their capabilities. Successful implementation of a project office depends more on initial staffing than any other factor. Make sure that what you are being asked to accomplish is consistent with your staff size and their abilities.
A project charter is an essential document for achieving consensus on the problem/opportunity, business objectives, scope and structure of a project. An organizational charter accomplishes this for a PMO. The PMO charter defines and communicates the organizational mandate for the PMO to exist. It describes the services provided, how to engage the PMO, and the PMO’s authority in areas such as reporting and standards compliance. “Organizational politics aside, the greatest challenge that most organizations face is to define what the PMO’s purpose should be, and why it is being created.” 
Here are a few topics that you want to make sure to address in a PMO charter:
Organizational History If starting a PMO has been tried before and failed, it is important to discuss what factors contributed to the failure and what is going to be done differently this time. Alternatively, if there are other PMOs within the company that are successful, this section can be used to discuss how lessons learned from them will be applied to this startup.
One last thought; in addition to a PMO charter it is a good idea to prepare a “sales brochure” that describes the services offered by the PMO and how to contract with the PMO for service delivery. The brochure is a good focus for discussion and a good “leave-behind” when meeting with prospective users of the PMO services.