At the Oscars, the award for Original Song (and Original Score) is just what it sounds like — music written specifically for a film, rather than for something else.
For instance, Jennifer Lawrence's 2014 performance of "The Hanging Tree" from the third Hunger Games movie was ineligible, because its lyrics were written by Suzanne Collins for the Hunger Games book series. The music was original to the movie, but that's not enough.
Original songs must be heard within the movie itself, or as the first song during the closing credits. They must also be "important" to the film they appear in — which often means that songs by prominent artists are nevertheless snubbed, because they don't play during a major montage or musical number.
The recent trend in the category has actually been to move away from songs that play over the closing credits. The 2017 nominees offered good examples of how the category has changed. Two songs were nominated from La La Land, an original musical, and both of those songs play during key emotional moments in the film. The same went for the songs nominated from Moana and Trolls, which are both animated musicals. (All of Moana’s songs are original; Trolls mostly uses songs not written for the film.)
The final 2017 nominee, from Jim: The James Foley Story, played over that film’s closing credits but highlights another interesting trend in this category — nominees from documentaries. Fully two of five nominees in 2016 were from documentaries, and in 2007, An Inconvenient Truth became the first documentary to win an Oscar in a category outside of the documentary feature or short categories, thanks to its win for Original Song, though it’s still the only documentary to win in this category.
The 2018 nominees don’t offer as stark a contrast, but they still feature two songs from musicals (the live-action Greatest Showman and the animated Coco) and a song from Call Me By Your Name that plays over a key montage around the film’s midpoint (as opposed to a similar song that plays over the film’s end). The other two songs — from Mudbound and Marshall — are more traditional “over credits” songs.
This is also a category that often boasts some degree of star power. This year, Mary J. Blige is nominated for her song from Mudbound, and indie favorite Sufjan Stevens wrote the song from Call Me By Your Name. If you’re a musician hoping to win an Oscar, Original Song is often your best bet. Everybody from Bruce Springsteen (“Streets of Philadelphia” from 1993’s Philadelphia) to Eminem (“Lose Yourself” from 2002’s 8 Mile) to John Legend (“Glory” from 2014’s Selma) has an Oscar thanks to this category.
The 2018 lineup offers one other interesting bit of trivia: the famed songwriter Diane Warren received her ninth nomination for co-writing the song for Marshall, making her the most nominated living songwriter without a win in this category. Unfortunately for her, the songs from Coco and Greatest Showman are favored here. Both are from songwriting teams who have already won — Coco’s Kristen Andersen-Lopez and Robert Lopez wrote “Let It Go” from Frozen, the winner in 2014, while Greatest Showman’s Benj Pasek and Justin Paul wrote “City of Stars” from La La Land, which won in 2017.