While I was working on Mac I never really liked OmniFocus. Then I used Taskpaper, then taskpaper-vim, then vim-orgmode.

Taskpaper is almost right but orgmode has few additions that I really need. Vim-orgmode works better than original orgmode on emacs for me.

In short try to pick something that can be easily migrated unless you want to be tied in ecosystem. There are ogmode clients for iPhone and Android, haven't checked them because I don't need them.

https://www.taskpaper.com/ https://github.com/davidoc/taskpaper.vim https://github.com/jceb/vim-orgmode

Taskpaper looks like a close alternative. If you are also looking for a web alternative to OmniFocus, the closest thing I've found so far is https://www.taskcade.com