OTA is basically a flag, or a tag that you don’t get to name. You could create a tag called #ota to serve the exact same purpose. So I look at OTA as a flag that I have easy access to, but it doesn’t necessarily have a specific meaning for me.
The primary benefit of OTA vs tags imo is easy access – there’s a keyboard shortcut to toggle it which you don’t get with tags, you can add / remove multiple notes at once, and the OTA saved search appears at the top of the overview. So that’s why I prefer to use tags to build up coarse-grained collections of notes, and then use OTA to focus on a smaller set of notes within / from those collections.
Tags, on the other hand, provide a second axis with their parameter. So you can create tags like
#WIP(important) giving you a way to find all WIP notes in a single #WIP search, or zoom in on urgent or important WIP notes by searching for
#WIP(important). Or my favorite, adding dates to tags like
#due(3 days) and setting up a saved search to see anything overdue, due in the next week, due in the next month, etc.
Personally I typically treat OTA as a sort of group of items to focus on for today or even the next work session. I don’t want something lingering OTA for more than a day. So I find tags are really good for these kinds of open loops. I’ll add a tag called #WIP and then do a saved search for notes matching those tags. So I can look at any #WIP notes and add them to the agenda for the day, along with any other notes that I want on the Agenda (I frequently have notes on the Agenda that are purely for quick reference purposes).
That said, you can come at it from the other side – use OTA to flag everything relevant for the day (or week…) and then create a #focus tag to pick out specific notes to focus on for a specific time period. I’ve done both, and am happy to change approach on a whim.