"Once I Loved" is one of Jobim's nicest Bossas, that is obscured by the more famous songs like "Girl from Ipanema" and "Wave". Like all Jobim songs, it is brilliantly crafted with interesting harmonies and graceful melodies.
This is such a natural melody that is relatively easy to solo on this tune. The only real complication is that it is a different song form than the usual 12-bar blues or 32-bar AABA or ABAB song. I would call this song form AAB with a tag. The A sections are 8 bars as usual. The B section isn't really a "bridge" in the sense of connecting two parts of the song. The B section simply begins the second half of the song. The B runs 8 bars, which is natural enough. Then the last 4 bars of that B section are played again (the "tag"). That gives us a chorus of 28 bars, not the typical 32. It flows so naturally, the audience doesn't miss those 4 bars at all. But the musician must know the difference.