Three problems with tags in OneNote Online
If you're a user of OneNote Online and OneNote desktop you'll have noticed the inconsistency with which tags have been implemented between the two platforms. Let's take a look at some of the differences between the two you need to keep in mind.
The desktop version of OneNote supports custom tags (creating your own). This is a powerful feature giving you control over how your information is marked and accessed quickly. Problem is custom tags do not carry over to OneNote Online at this time. If you even change around the order of tags on the desktop, these changes won't be reflected online. Personally I find this a huge limitation to the functionality between the two platforms (since I use both equally as often.) At this time the only option is, if you are using both platforms, to limit yourself to the tags and order available online. This way you can be sure you'll have the same tag experience on both platforms.
On the desktop application you can search across pages, sections, and notebooks for specific tags and generate lists and pages for easy reference. Unfortunately this functionality is not available online. I think this is something they should be able to implement in the near future, but then again when I look at the limitations of searching online notebooks I have to wonder. Online, tags are useful at the page level, but aggregation beyond that is just out of reach.
On the desktop you can access tags while writing by using hotkey combinations of Ctrl-1 through Ctrl-9. You can do the same online (though the application doesn't tell you that fact in any obvious way). You are also limited to the default order of the tags and their hotkeys. This can cause a significant issue if you customize your tags at the desktop level as I mentioned in the first section.
OneNote doesn't really support text tags in the classic sense and seems to have a significant blind spot when it comes to special characters. If you're used to using a hashtag (#) for your tags, take note that OneNote will ignore the hashtag on a search. So if your tag is #banana, every instance of the word "banana" will be returned when you search for "#banana". I haven't found a away around this issue as of yet, but when I do or when Microsoft fixes this oversight I'll be sure to let you know.
What can you do?
If you're using OneNote Online and OneNote desktop, just remember to stick to the functionality available online for now. If you're going to be using both, remember many of the powerful aspects of tags just aren't there yet online and you could be setting yourself and your team up for a disappointment. Hopefully soon Microsoft will unify the tag functionality between both platforms. Only time will tell.