If you collapse the sub-topics, then select your rows, when you do Organize -> Sort Selected Rows, it will only sort the level that shows.
Also, if you set a focus, it will be restricted to the focus. Unfortunately it looks like you can’t sort when using a filter.
I have long wanted OO to expose some of the outline metadata to make certain things easier. For example, I’d like to be able to limit a search to a particular section of the outline, or only down to a particular level. Filters do limit the search to those rows matching the filter, but in keeping with the notion of exposing metadata, if match types included less-than row level, greater-than row level, or equal-to row level, we could limit searches that way.
And I’m STILL waiting for regular expressions on filters.