What is an OPAC (Online Public Access Catalogue)?

OPAC, unpacked

The acronym stands for Online Public Access Catalogue. Every ILS keeps a "back office" side, used by staff to catalogue items, process loans, and manage patron accounts, and a public side, used by whoever the library serves to search the collection without needing any staff login. That public side is the OPAC. It's built on exactly the same underlying data as the staff catalogue — if a librarian adds a new title, it appears in the OPAC automatically, usually within moments.

What patrons can actually do in an OPAC

A basic OPAC lets someone search by title, author, subject or keyword and see whether an item is on the shelf or out on loan. Most modern OPACs go well beyond that baseline:

• Placing holds on items currently checked out, so they're notified when it's back.
• Renewing loans themselves, without a phone call or a desk visit, provided nobody else is waiting.
• Viewing their own account — current loans, due dates, and any fines.
• Browsing curated lists — new arrivals, staff picks, or themed collections the library has put together.
• Advanced search — filtering by format, location, availability, or custom fields specific to that collection.

Why the OPAC's branding matters

Because it's the only part of the system the public ever sees, the OPAC is effectively the library's digital front door. Many modern systems let the library apply its own logo, colours and footer so the portal feels like an extension of the organisation rather than a generic third-party tool — something that matters more for public libraries and membership organisations than for internal staff-only catalogues.

What good OPAC search actually looks like

Not all OPACs search equally well, and this is one area where it's worth testing a system hands-on rather than trusting a feature list. A capable OPAC typically supports keyword search across multiple fields at once (title, author, subject, notes), Boolean operators for narrowing results, and faceted filtering — letting a patron narrow "results for 'gardening'" down by format, availability, or location with a click rather than a new search. Spelling tolerance and "did you mean" suggestions matter more than they sound, since a chunk of real-world searches contain typos. If a system's demo search only handles exact title matches well, that's worth probing further before you commit.

Accessibility and mobile use

Because the OPAC is public-facing, accessibility isn't optional the way it might be for a staff-only tool. Look for keyboard navigability, screen-reader-friendly markup, and sufficient colour contrast, particularly if your organisation is a public body with legal accessibility obligations. A growing share of OPAC searches now happen on a phone rather than a desktop, especially for public libraries, so a genuinely responsive layout (not just a shrunk-down desktop page) is worth checking during any product trial.

OPAC vs "discovery layer"

You may also come across the term discovery layer (or discovery service) — a more advanced search interface, sometimes from a different vendor, that sits in front of the OPAC and can search across multiple sources at once, such as several branch catalogues or subscription databases. A discovery layer is an optional add-on for larger or more complex institutions; most libraries run perfectly well on their ILS's built-in OPAC without one.

How the OPAC relates to the wider ILS

It's easy to think of the OPAC as a separate product, but it isn't — it's one module of the ILS, reading from the same catalogue and circulation data that staff use every day. If you're still getting oriented on the bigger picture, see what is an ILS? for how the OPAC fits alongside cataloguing, circulation and patron management.