3C = Third Cambridge Catalog (of radio soures that are bright at low freqeuncies). This got many of the local radio galaxies and quasars as well as galactic sources, so these names are still used a good bit. 3C 273 is the brightest quasar, 3C 405 is the radio galaxy Cygnus A, 3C 120 is well-known only by that name. 3C 75 is a binary pair of galaxies each with twin radio jets. (The first two catalogs in teh series were much less reliable and immediately dropped from usefulness). There are also 4C, 5C, 6C surveys which are much deeper cover progressively less of the sky.

OH, OQ... are radio designations from a survey done at Ohio State University almost 50 years ago. The second latter denotes declination bands around the sky.

PG - Palomar-Green, a survey of the sky for blue objects done by Richard Green for his dissertation. This captured a fair number of brightish QSOs that had been hitherto unknown (thus giving clues about how complete the surveys might be), as well as nearby white dwarfs and cataclysmic variable stars.

PHL = Palomar Haro-Luyten, a search for faint blue stars at high galactic latitudes which also netted some things which eventually turned out to be quasars (PHL 5200 with its spectacular absorption lines from an outflow may be the best known).