Others had already mentioned Your Song and My Song having similar melody, and The Realist and The Dreamer being a loop. Here are more from ONF:

• their version of It’s Raining (original song by Rain) samples their own song Complete

• Ugly Dance interpolates Beautiful Beautiful

• If We Dream and Alarm are connected

• 첫 사랑의 법칙 (Happily Never After) and 첫 키스의 법칙 Belle Epoque - The Korean titles translate as "Rule of First..."

• The intro of Whistle is used at the beginning of 'Your Song' MV