FMOTP
It seems to me there are a couple of reasons for a band to release an EP. The first is to give curious listeners an introduction by offering a taste of the band's music. The other is to give existing fans of the band some extra material or songs to satisfy them between albums. I think the OBLIVION EP succeeds brilliantly at the first task. I really enjoy Mastodon's music, although I haven't listened to many of their CD's yet.
This EP is a good introduction to the band's sound. I would pick the title track and the last one, "Colony of Birchmen", as my favorite songs here. Since they are listed as Mastodon "Top Tracks" on YouTube, these two seem to be among the band's most popular and well-known songs. They just strike me as more creative than the middle two songs, although all four are very good.
One way to judge a band is to compare it to similar bands in the same category. Mastodon are often compared to Baroness. Although I can respect what Baroness does, Mastodon are stronger songwriters IMHO. I won't discuss the contributions of individual band members at length; I usually don't care to do so in my reviews. I will reiterate what others have said about the drummer, Brann Dailor. His handling of the band's often complex music is a major part of Mastodon's success. He is without a doubt one of the best drummers in progressive metal music (using a broad definition of that phrase). To wrap things up, I'm giving four and 1/2 stars to this excellent EP.