Huawei's own operating system seems nailed on for October when it will be revealed running on the new Mate 30 series.
This is pretty much what we've suspected ever since Huawei's blacklisting by The US Government. Although there's a reprieve on the ban currently, this is only set to run until August.
We've known for a few months that Huawei was developing its own software as a backup and a trademark filing has been spotted for Ark OS, though it's thought to be known as HongMeng OS in China. It may be announced earlier than the phone itself, Global Times reports. We've also heard it may be called Oak OS, but this could be a mispronunciation and there is no trademark filing for that name.
It'll be based on the open source version of Android, so don't expect it to look vastly different to Huawei's existing software - which is a skin running on top of Android anyway.
However, there won't be any Google apps, though it will surely be possible to install them yourself in the same way you'd install them on other Android phones that don't have them.
The Mate series is normally announced around October and it normally runs on the chipset that Huawei announces at in early September.
We are expecting Huawei to announce the 5G-incorporating Kirin 9990 at IFA 2019, but with ARM also suspending business with Huawei currently, we're not sure how it can do this.
Although we're guessing that the chipset will have been designed some time ago, so maybe the work had already been completed from the ARM-side by the time the ban came in.
Last week Google warned the US Government that there was potential for Huawei's version of Android to pose a security risk. In other words, Google feels it would be much better if the US Government opted to leave tech companies in control of the situation and lifted the ban.
Edit 12 June: Added clarification between Ark and Oak names for the new OS