I guess it all goes quiet now until the results come through
The TSA results used to come through the night before Oxford offers were made, not before interview. I know DD had to apply to get her mark and they don't release marks for the essay part. She was comfortably within the top 15% which definitely helped her to secure an interview, but there isn't a firm cut off point.
Your DC should hear if they are being asked to interview around 2 weeks before. DD was invited to a UNIQ interview workshop at Oxford in the October half term, but they made it clear that everyone who had been on UNIQ was invited and it wasn't linked to whether or not you would be.
Her school gave her a practice interview, but it was with someone who had studied a random subject at Brookes and bore no resemblance to her real PPE interviews. They covered everything from optimal pricing strategies for car parks, setting tax rates under a given scenario, the Kardashians and Dr Who style time travel. Nothing could really prepare you for that!
She hadn't studied any of the 3 disciplines at A level, so no background knowledge was expected.