Zane Kleinberg (via Hacker News):
OldOS is a testament to the days of yesteryear, showcasing what iOS once was ten years ago. The ethos of the app is to merge the technologies of today with a pixel-perfect recreation of the user experience of the past. The vast majority of apps in OldOS are fully functional — meaning they seamlessly integrate with the data on your phone to deliver a live, emulator-esque experience. What does this mean? Well, you can play your music in iPod, get directions in Maps, surf the web in Safari, view the current weather in Weather, and much more. By the same token, no shortcuts were taken in fully fleshing out the operating system. You can change your background, adjust settings, search apps, et cetera. There are a few apps still not ready for primetime but don't worry, they're coming soon.
With OldOS, you no longer need to worry about securing a legacy iPhone to experience nostalgia — it's available on your daily driver.
Once you get past the surface aesthetic differences, it’s also interesting as a way to remember how many little things iOS has added over the years. iOS is so much richer now. You couldn’t do anything in list views back then. E.g., if you wanted to delete a note in Notes, you had to open the note and tap the Trash button. In a view hierarchy, you couldn’t go back just by swiping from the left edge of the display — you had to tap the Back button in the navigation bar at the top of the display.