The county's Public Health boss is particularly worried about an increase in cases in the over 60s.
Twenty million more people are living under Tier 4 restrictions in England this morning - after another day with more than 50,000 new cases of coronavirus confirmed across the UK.
Education Secretary Gavin Williamson says he's "confident" there won't be another national lockdown.
It means a quiet New Year, with people in Oxfordshire being advised to stay at home.
Ansaf Azhar, the County Council's Director for Public Health, said: "We have had some good news with the new Oxford vaccine being approved and we know that a number of people in Oxfordshire have now begun the process of being vaccinated. However we cannot allow this to tempt us in to relaxing - we still have tough Winter weeks ahead of us before the vaccines will start to have an impact.
"We know that the new variant of COVID-19 spreads extremely quickly and these rules were applied for our own safety and protection. The numbers of cases across the county have risen very steeply and we have also seen sharp rises in hospital admissions.
"Viruses always thrive in Winter and we can clearly see how aggressively COVID-19 can spread if we let our guard down even marginally. We have seen during 2020 the damage that this virus can do when people are in close contact with each other."
The new variant is significantly more transmissible and is becoming the dominant strain across the country - more places have now joined Oxfordshire and our neighbouring areas in Tier 4.
The latest data shows that the virus is continuing to spread rapidly locally.
The weekly rate currently stands at 338.2 per 100,000. The rise is particularly concerning among the over 60s, where cases are continuing to rise in all parts of the county. Hospital admissions are increasing rapidly as a result.
The highest rate in Oxfordshire is in Cherwell which stands at 475.7 cases per 100,000.
Ansaf added: "Unless we all take personal responsibility as individuals and families in the early weeks of 2021, we will create a deeply deteriorating situation in January. This means putting even more pressure on our NHS at its busiest time of year.
"I continue to urge everyone to adhere to the new tier 4 rules and to take every single precaution. We need to protect ourselves, our communities our families and our work colleagues in the coming weeks before the vaccine is widely enough in circulation to begin to have an effect. Please do not let your guard slip."
Tier 4 is the highest alert level in the national four-tier system, and it means that Oxfordshire residents must stay at home, except for a limited number of purposes. These include essential work that cannot be done from home, buying food, taking exercise, or getting medical treatment.