"___-la-la!"

OOH

Think of the playful sound you might sing before "la-la" in a cheerful tune.

This clue mimics the playful, nonsensical refrain from the 1964 hit song "I Want to Hold Your Hand" by The Beatles, where the lyrics go "Oh-la-la-la, oh-la-la-la!" The blank represents the "Oh" sound, often spelled as "Ooh" in informal writing to match the song’s cheerful, exaggerated tone. Since crosswords favor short, punchy answers, "Ooh" fits perfectly as the three-letter exclamation that completes the familiar phrase.