The first COO is one of the hardest hires a founder makes, because the role means something different in every company.

When

Hire when the founders spend more time coordinating than deciding, and when the operating rhythm depends on one person remembering to run it.

Write down which decisions the COO will own before you write the job description.

Who

Look for someone who has run the stage you are about to enter, not the stage you are in today. And make sure they complement the CEO rather than mirror them.