Nearly four in 10 close contacts of people in the county testing positive for Covid-19 were not reached by the end of September, official figures show.
Oxfordshire also saw its largest weekly increase in the number of positive coronavirus cases transferred to the test and trace service at the end of September, figures show.
Data from the Department for Health and Social care shows 1,265 people who tested positive for Covid-19 in Oxfordshire were transferred to the Test and Trace service between May 28 and September 30.
That was 184 more than the total transferred up to September 23 – the largest weekly increase since comparable local figures were first published in August.
Test and Trace asks these patients to give details for anyone they were in close contact with in the 48 hours before their symptoms started.
In Oxfordshire, 3,360 close contacts were identified but just 63.7 per cent of those were reached by contact tracers in the four month period, meaning 1,220 people were not contacted or did not respond.
That was only slightly up from the 63.2 per cent reached in the period to September 23.
Across England, 51,500 people tested positive for Covid-19 in the seven days to September 30, making it the highest total since the regime launched in May.
However, only 67 per cent (34,500) were transferred into the contact tracing system.
Baroness Dido Harding, who is interim executive chairwoman of the National Institute for Health Protection, said demand for tests is rising with the growing number of cases.
She said: “We are working hard to increase testing capacity to meet that demand and improve turnaround times for tests.
“We have now opened 500 test sites across the UK, an extraordinary achievement.”