Long-click on a note in the list of notes. This opens a context menu with the "Edit tags" command.
Once you have added tags to your notes, a selection field will appear at the top of the list of notes so that you can select notes with a specific tag. (You may have to exit OI Notepad with the back button and launch it again for the selection box to appear).
While viewing notes with a specific tag, adding a new note will automatically add this tag to the note.
Select "All notes" to view all notes again.