OmniOutliner Alternatives for Mac OS X

wikidPad is a free and open-source wiki-like notebook application that allows you to store your thoughts, notes, and snippets of information in a highly organized and cross-referenced manner. Its strength lies in the effortless creation of links between pages, making it easy to navigate and connect related ideas.

NixNote is an open-source, cross-platform desktop client for Evernote, offering a familiar interface and offline access to your notes.

KeepNote is a versatile note-taking application designed for organizing information in a hierarchical structure. Available on Windows, Linux, and macOS X, it features robust organization tools, including a tree-like outline, wiki-like linking, and support for embedded files and images. Although discontinued, its local storage and rich features make it a compelling tool for detailed note-taking and research.

Org-mode is a powerful and flexible mode for GNU Emacs, designed for note-taking, project planning, to-do list management, and authoring documents. It utilizes simple plain text files with intuitive markup.

Piggydb is an open-source web notebook application designed for building and managing knowledge bases, either individually or collaboratively. It emphasizes structured organization through hierarchical tagging and concept mapping.

Moo.do is a collaborative task management and outlining tool designed to consolidate your work, notes, and tasks into a single, flexible interface.

Germ.io is a collaborative idea management and project planning tool that helps teams capture, refine, and transform initial concepts into structured and actionable project plans.

Manuskript is a free and open-source writing software designed specifically for authors. It provides a structured environment to plan, outline, and write novels and other complex documents, featuring tools for character tracking, plot management, and a distraction-free interface.

Goalton.com is a comprehensive platform designed for effective goal setting, task management, and project execution. It provides a flexible and hierarchical structure to organize your work, combining various views like Kanban boards, mind maps, and outlines to suit different workflows and collaboration needs for individuals and teams.

Notion is an all-in-one workspace that combines notes, documents, wikis, project management, and databases into a single, highly flexible platform. It empowers teams and individuals to manage knowledge, plan projects, track tasks, and collaborate seamlessly, adapting to nearly any workflow or organizational structure.