With all due respect, to those who recommend “easing into bluegrass”, I say, “How dast ye!”. If you wanna listen to bluegrass, then listen to bluegrass. If you’re meant to like it, the songs you hear will begin to grow on you soon enough. That said, I recommend the following:
Bill Monroe and the Bluegrass Boys - “The High Lonesome Sound”: As previously mentioned, Bill Monroe was the first well known bluegrass artist, the genre’s name being taken from his band’s name. Features fine singing and playing all around, and Monroe’s fiery mandolin playing.
Flatt and Scrugggs - “Blue Ridge Cabin Home”: F&S were part of Bill Monroe’s band, but split with him and made many great sides during the fifties. This compilation is my favorite. Scruggs, of course, popularized a three-finger style of picking the banjo, emulated by many banjo players since. Also featured is the dobro playing of Josh Graves, and instrument first added and popularized by F&S.
Don Reno and Red Smiley - “20 Bluegrass Originals”: Reno was also a magnificent banjor player, but played in a more flashy style, influenced by the fact that he was also a fine guitarist. The songs feature Smiley as lead vocalist, with the beautiful high tenor voice and good humor supplied by Reno.
The Stanley Brothers - “Complete Columbia Recordings”: Ralph Stanley is of course on the “O Brother Where Art Thou” CD mentioned in the OP, but in the late forties up until his brother Carter died in the 1966, performed with him in a style somewhat close the Bill Monroe, but influenced more by the mountain songs of Appalachia where they grew up. Ralph’s banjo playing was straightforward and simple, but very powerful.
There are so many more, but the above will get you started. Without going into detail about the following, if you like the above, try these artists also.
The Osborne Brothers
Jim and Jesse
J.D. Crowe and the New South
Doyle Lawson and Quicksilver
Skaggs and Rice