So according to the release the new features are:

-Picture in picture (dual app onscreen)

-Notification dots and "force-touch-like-feature" (my words, not Google's) to preview

-Google Play Protect (an antivirus) [0]

-Minimize unintentional overuse of battery from apps in the background

-Faster boot speed

-Support forAndroid Instant Apps

-Over 60 new emoji (sic)

Under the hood stuff, not in the release (thanks u/izakus)

- Project Treble that splits hardware HAL layer away from the OS layer. Plan being that this allows updating the OS without having to update the whole hardware driver stack (and thus avoid being dependant on Qualcomms of the world to update their proprietary blobs for each release)
- GPU drivers are now updatable via Play Store (by OEMs)

More new features listed on the main Oreo page [2]:

-Accessibility button: Allows you to quickly access from the navigation bar accessibility features, like magnification, and functionality within accessibility services, like Select to Speak.

-Accessibility volume: Accessibility services can optimize the audio experience for users with disabilities.

-Adaptive icons: Developers can now provide a full-bleed square shaped icon and OEMs will mask the icon to ensure intra-device consistency.

-Ambient screen: Highlights the incoming notification with larger font, highlighted app name and immediate access to actions.

-Background execution limits: More control over how apps run in the background for better overall system performance.

-Background location limits: Limits the frequency of location updates in the background for better overall system health.

-Deep color: Enables applications to render richer visual content with more vibrant colors and subtler gradients. Supports full color management which allows applications to render images in the format and quality they were intended.

-Downloadable fonts: Applications no longer need to bundle custom fonts, which helps reduce their size.

-Install unknown apps: Hostile downloader apps can't operate without permission; users now permit the installation of APKs per-source.

-Integrated printing support: Compatible with all Mopria-certified printers, which make up 97% of printers sold worldwide.

-Linkable files: API that allows you to share files across the Internet via web links.

-Native C/C++ API for high-performance audio: API function for high-performance audio including Native C/C++ audio API.

-Notification categories: More granular and consistent control over which notifications can appear and how intrusive they are.

-Notification snoozing: Lets users hide notifications for a period of time, similar to Inbox snoozing.

-Pointer capture: Pointer capture allows the app to capture all mouse input.

-Project Treble: The biggest change to the foundations of Android to date: a modular architecture that makes it easier and faster for hardware makers to deliver Android updates.

-TextView autosizing: Developers can now let the size of their text expand or contract automatically based on the size and characteristics of the TextView, making it much easier to optimize the text size on different screens or with dynamic content.

-Tooltips: Support for tooltips (small popup windows with descriptive text) for views and menu items.

-Wi-Fi Assistant: Auto-connects you to high quality open WiFi and secures your connection with a VPN back to Google.

[0] https://www.android.com/play-protect/

[1] https://developer.android.com/topic/instant-apps/index.html

[2] https://www.android.com/versions/oreo-8-0/