Oba is a big point for making or preventing a moyo or near territory. Typically it is a large extension on the side or a wedge. While the literal translation is big point (or area), there are other kinds of big plays.
See Kyuuba before Ooba.
During the fuseki of a professional game, the players often spend a lot of thinking time to try and grab the last big point on the board (oba no tedomari).
If White adopts a mimic go strategy, Black can take the last oba by playing move 11 at a (tengen).
Sebastian: Not sure how this example illustrates the point. Why not move it to MirrorGo?
Charles It illustrates the tedomari concept rather than the oba concept. Perhaps a better example can be found.