You can create DOOH audiences from the 'Audience Builder' section on the Properties area of any of your DOOH campaigns.

These audiences can be targeted in mediasmart campaigns to run mobile ads, CTV ads on the households of DOOH viewers, or perform drive-to-store campaigns over those users, among other use cases.

You just need to either:

Choose an existing audience to populate,
Create a new DOOH audience to build

The configuration of the Audience Builder for DOOH is different from other types of campaigns, as it will only let you create Mobile audiences in a 250m radius around the DOOH screens you are targeting in your campaign.

Configure or Edit a DOOH campaign