Oberon is both a programming language in the Pascal-Modula tradition and a modern operating system for single-user personal workstations. Its highlights are (a) an integrated and modular programming environment, and (b) a versatile textual user interface. Oberon System 3 is an evolution of the original Oberon system that features (c) a built-in management of persistent objects, and (d) a sophisticated component framework called Gadgets for the interactive and programmed construction of visual objects, model objects, and entire graphical user interfaces. Henceforth in this text, we shall use the terms Oberon and Oberon system synonymously with Oberon System 3, expressed in the language Oberon. The main objective of this paper is a systematic presentation of the complete Oberon system architecture whose cornerstones are several well-defined, separate, and extensible hierarchies that interact harmoniously. In detail, the topics dealt with in this paper are (a) modular structures and their relation to class libraries, (b) a suitable kernel support for the management of persistent objects, and (c) concepts and structure of the Gadgets package. In particular, we make an attempt to classify the different activities in the relatively new and complex field of component construction. In a brief interlude, we comment on a current study of some kind of concurrent objects, called active objects.