A2 (formerly named Active Object System (AOS),[2] and then Bluebottle) is a modular, object-oriented operating system with unconventional features including automatic garbage-collected memory management, and a zooming user interface.

A2's design allows developing efficient systems based on active objects which run directly on hardware, with no mediating interpreter or virtual machine.

Other differences between A2 and more mainstream operating systems is a very minimalist design, completely implemented in a type-safe language, with automatic memory management, combined with a powerful and flexible set of primitives (at the level of programming language and runtime system) for synchronising access to the internal properties of objects in competing execution contexts.

Above the kernel layer, A2 provides a flexible set of modules providing unified abstractions for devices and services, such as file systems, user interfaces, computer network connections, media codecs, etc.

Like Oberon, though, its user interface supports a point and click interface metaphor to execute commands directly from text, similar to clicking hyperlinks in a web browser.