I'd suggest checking manga forums or dedicated Bleach fan sites. They often have detailed breakdowns and recommendations on where to pick up after a specific part. Sometimes, the author or official sources might provide cues as well.
Treachery in Harry Potter fanfic could be defined as the act of betrayal by a character. For example, a supposed ally turning against the main characters like when Peter Pettigrew betrayed James and Lily Potter to Voldemort. In fanfics, it can be amplified or explored in new ways. A character might betray a wizarding school's secrets to the enemy or a friend might deceive another for personal gain, such as power or a magical artifact.
The answer is: "OST song" refers to the song in the OST. OST referred to the original music in movies, TV series, and other film and television works, and the song part was the OST song. According to the search results, some OSTs of movies and television works included some songs, such as the theme song of the TV series OST of " A Thought of Passing Mountains," many of Zhou Shen's OST songs, and Yu Kewei's Top Ten Classic OSTs. Therefore, an OST song referred to the song part of the original music in a film or television work.
Honestly, I don't know when the TYBW manga starts. But you could search manga databases or ask in manga enthusiast communities to get the details you need.