OKRs: Turning Priorities into an Objective (or Objectives)
An Objective for an OKR set (the term that's often used for an Objective and its 3–5 Key Results) is not the same as the priority you've identified. Actually defining the Objective, and then finding the right words to phrase it, is an important and distinctive step.
The OKR Objective should probably be set by the CEO, most likely in conjunction with the senior management team. However, in an agency that has a motivated and capable team with strong autonomy, the best Objectives will be informed by the team — they're the ones who are closest to the action, to your customers and in delivering your services, and their sense of what the company should focus on will be especially sensitive.