The county council's urging under 30s to get tested at one of three walk-in units.
More than a thousand young people took part in PCR testing at temporary facilities in Oxford over the weekend.
Now, the county council says the pop-up sites are back open today, until Sunday, so that more 18-29-year-olds can get a test.
Cases in Oxford jumped from 296 to 946 this week (ending 2 July), with 80% falling in this age group.
There's currently 620.5 coronavirus cases per 100,000 people - this compares to a rate of 56.4 per 100,000 in the city as recently as June 4th.
The walk-in mobile testing units are open from 10am - 8pm (no appointment is needed) at:
- South Park (St Clement's end)
- The Blavatnik School of Government, Walton Street (on the right hand side of the front of the building)
- Manzil Way Gardens, Cowley Road.
The testing centres at Oxford Brookes University and Osney Lane are also open every day 8am - 8pm, including for those without symptoms, but people need to book an appointment on gov.uk.
Ansaf Azhar, Oxfordshire County Council's Director for Public Health, said: "Cases in Oxford in the last seven days have risen over three-fold and this comes on top of large rises in recent weeks. The overall rise in cases over the last month has been exceptionally steep.
"Oxford is now a COVID hotspot for the whole of England and we have seen nothing like this since the big spike in cases at Christmas. We must do all we can to get on top of this situation together working with central government, which is very much aware of the rise we are experiencing.
"Around 80 per cent of our cases in Oxford are 18 - 29 year olds so we are urging anyone in this age group to get a PCR test this week. Thank you to all those who took part in testing at the weekend - this was a fantastic effort - but please help us spread the word to your friends, family and colleagues of the same age to come along and do the same."
Ansaf said cases around the county 'are at lower levels but they too are rising', adding that rates in the over 60s are still comparatively low 'but they have risen four-fold over the last week.'
He said: "As the government announced yesterday, we are on track for society to reopen fully on 19 July but regular testing and completing the vaccination programme remain the nation's long-term plan for living with COVID-19. If you haven't already, please take up your offer of a first vaccination."
To date, over 765,500 COVID-19 vaccinations have been delivered in Oxfordshire since the start of the roll-out.
Councillor Liz Leffman, Leader of Oxfordshire County Council, said: "Oxford is now top of national league tables for COVID and it is a situation we need to tackle collectively as individuals, friends, colleagues and families. The national situation is clearly moving from rules and roadmaps to personal responsibility and common sense.
"With this spike in cases in mind, I would therefore urge everyone to continue to do the things that are tried and tested in terms of keeping us safer - keep a distance, wear a mask, wash your hands, socialise outdoors where possible and keep indoor spaces ventilated. Let's confront this rise in cases together and drive the numbers back down."
Case rates across Oxfordshire per 100,000 population:
- Cherwell: 176.7
- Oxford: 620.5
- South Oxfordshire: 206.3
- Vale of White Horse: 147.8
- West Oxfordshire: 139.2