Thank you for confirming, ok.
When a cat exhibits behaviour like this there can be several triggers including underlying itchiness, urinary tract disease, a source of abdominal or joint pain or stress-related and behavioural issues.
If the vet is confident that they've ruled out any physical causes for Oscar's signs and his skin is calm and settled, it may well be a behavioural issue.
We can only really confirm this by ruling out all other causes (which usually means a urine analysis and comprehensive skin work up).
A blood test, as you mention, wouldn't usually be helpful in this situation.
If there is no obvious medical cause, we need to look to his routine and home life for any source of stress.
Does he share the home with other pets?Did anything change when the behaviour started e.g. a house move or new person in the home?Does he tend to be skittish or anxious in general?