Is it OK to have HIFU on my face after PDO threads?

It would not be advisable to have Hifu after PDO threads. You would need to wait for these to be fully dissolved (1 year after threads done) 
In the meantime, to have lifting and tightening, you could look at Botox and Fillers to help with this. 
There are a few things to note: 
Ultherapy is the best form of Hifu. It does not cause fat loss or adverse reactions as it is a guided treatment. It is the only treatment on the market with has a guidance screen, meaning the practitioner can see what they are treating and see the depths of the skin. Without this, the practitioner does not know if they are treating tissue, fat or even bone. Therefore, you can suffer from bone bruising and even fat loss, causing a loss of volume. When considering injectables, ensure you are only using FDA regulated products, not just CE. Only see a medical practitioner (doctor) as they are trained extensively on both the anatomy of the face and dealing effectively with complications. Anyone can train in injectables, and it only requires a one day course to be able to inject so it really is essential you research who is doing your treatments. Further, over 180 fillers are available in the UK and only 8 of these are allowed in the USA. FDA approval for all treatments and injectables is a must. 

I hope this helps, and best of luck with your treatment.