You do not need certification to get into a PMO job. PMO is all about project management skills stemming from experience and expertise in the area. If you are considering HR PMO, then it is highly strategic in nature. It is prevalent in any area where projects/programs are being implemented. Most organizations run PMOs for various HR functions such as L&D, Operations, Shared Services, HR-IT, etc. You must have a thorough understanding of the vertical from a practical perspective and then work on projects within that department. You may collaborate with other coordinators located in different countries to standardize programs across all locations.
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