As per google search results; The Nothing Phone 3a and 3a Pro launched with Nothing OS 3.1, which is based on Android 15.
Nothing OS 3.5:
Nothing OS 3.5 is the next major version of the operating system, but it’s not currently available for the 3a series.
Future Updates:
While the 3a series launched with Nothing OS 3.1, Nothing provides a commitment to three Android upgrades and four years of security updates, so it will receive updates to newer versions of Nothing OS in the future.
The next major update for the 3a and 3a Pro is expected to include OS 4.0 with Android 16, which will likely arrive either later this year or in early 2026 — though this may vary depending on the system and region.
