Omni Notes is described as 'Note taking open-source application aimed to have both a simple interface but keeping smart behavior' and is a popular app in the Office & Productivity category. There are more than 100 alternatives to Omni Notes for a variety of platforms, including Android, Windows, Mac, Online / Web-based and iPhone. The best alternative is Joplin. It's not free, so if you're looking for a free alternative, you could try Standard Notes or Google Keep. Other great apps like Omni Notes are Microsoft OneNote (Freemium), Evernote (Freemium), Simplenote (Free, Open Source) and CherryTree (Free, Open Source).
Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, can be copied, tagged and modified either from the applications directly or from your own text editor.