The flute than the vocal african sounding scatting with the percussion backing. Slow addition of acoustic guitars, synths, brass, electric guitar, bass. Then the brilliant reprise of the opening theme but with thumping rhythm section and some great bass playing. The way the song just ends with everyone suddenly stopping leaving behind the percussion was a pretty ingenious move.
It's really masterful crescendo from Oldfield. I also have to admit this is one of the very few times in music where if the percussion was absent, the music would be significantly worsen.
Ommadawn Part One is Mike Oldfield greatest moment, although admitting part two does fall under the "pretty background music" (so perhaps Tubular Bells is better from start to finish).
I also think what makes Ommadawn Part One is probably one of the best side long prog suite. While the music is medley-esque, the transitions is one of the most smooth out of all the suites I've heard. When there is a sudden mood change due to change in arrangement, like the sudden sparse playful folk moment at the 6:57 mark with the piano and flute and then banjo section when before it was some slow proggish dreamy section. Mike Oldfield introduced the melodies that would dominate the next section but with the earlier arrangement/mood (6:10). So when the sudden mood change occurs, the music still feels continuous as the melodies are connected.
He would do this often where he will introduced the melodies prior to the mood change and changed in section.