I have been involved in a couple of startups.
From what I have seen, stakeholders and communication management are probably the most important skills in that environment... all too often startups are focused on the product, while they should be focused on the market and business model.
Agile Project Management is the obvious way to go for a startup, provided a high level of seniority is available.

Well said Stakeholder management and communication management are top challenges faced by PMO.

There are many articles on this subject you can read on pmzilla also