OOH can be bought two ways, and most sophisticated plans use both. Here's the practical difference.
Direct (guaranteed) buying
You book specific units for specific dates at agreed rates. You know exactly which faces will carry your creative, placement is guaranteed, and premium or iconic inventory (spectaculars, dominations, specific billboards) is only available this way.
Strengths: Certainty, unit-level selection, access to all formats including static and premium digital.
Tradeoffs: Committed dates and spend; changes after contract depend on media owner terms.
Programmatic buying
Digital OOH bought through automated channels: you define targeting, budget, and parameters, and impressions are delivered across eligible screens.
Strengths: Flexibility; start, pause, and adjust quickly; efficient for broad digital reach and shorter windows; lower commitment thresholds.
Tradeoffs: Digital screens only; you buy delivery against parameters rather than hand-picking every face; verification is playout-based rather than photo-based.
Can I combine them in one campaign?
Yes, and it's the recommended pattern for many plans: direct buys anchor the campaign with guaranteed, high-impact placements, while programmatic adds flexible digital frequency around them. Both run under one campaign with unified reporting.
Which should I use?
Specific units matter (iconic locations, exact placements, static formats): direct.
Speed and adjustability matter (short windows, testing, variable budgets): programmatic.
Broad awareness with impact moments: both, anchored by direct.
How does verification differ?
Direct static gets photo proof of posting; direct digital and programmatic get playout reporting. If photo-style verification is a stakeholder requirement, weight the plan toward direct inventory.