Oligarch is a term used in planet formation theory, to signify planetesimals that have grown larger than the majority, in some sense dominating the subsequent development into the resulting planets. Such theories necessarily have evolved, based on observation and computer simulations, and any "complete" theory needs means of creating both rocky planets and giant planets, the differing results implying some difference in the growth path.
A general theory is that at some early point, there are numerous planetesimals, which undergo a stage of rapid growth (runaway growth) when the larger ones are at least as likely to add more mass as the smaller ones, if not more. Some become significantly larger than the rest (the oligarchs), after which they grow more slowly (oligarchic growth aka oligarchic accretion), when smaller planetesimals impact/merge with them, and at the same time, some of the planetesimals are flung out of the system. Given the slower growth, this is presumed to be a much longer growth phase. The result is the smaller set of planets that eventually have a long life, such as the solar system's eight planets.