Health chiefs say 13 cases had been confirmed by Monday night but it was 'highly probable' there were more.
Anyone with coronavirus symptoms or anyone who has been in close contact with someone who has recently tested positive for the virus should get a test to see if they are infected — although appointments need to be booked online in advance.
Omicron was first identified less than two weeks ago but is believed to be the most transmissible Covid variant yet and experts are wary a surge in infections could overwhelm the NHS this winter.
Lucy Wightman, director of Public Health Northamptonshire, said: "We don't want people to panic but we asking them to exercise common sense and caution and where they may have started to be a bit complacent about wearing face coverings, who they meet, taking regular lateral flow tests and whether they've had their booster vaccination.
"We are acutely aware that people are fed up with this.
"At same time, with this new variant we don't fully understand how transmissible it is, the impact on somebody vulnerable or how effective our vaccination programme is against it.
"We don't know how serious this can be health-wise and what we do know is that of the cases we've identified nobody required hospital treatment.
"But the population we're dealing with is primarily young and healthy and not anybody with significant underlying health conditions.
"So that's not to say if others in wider community contracted Omicron they would also have mild symptoms.
"We're still dealing with small numbers and until we've seen more cases and how it affects different people with differing underlying conditions, we don't really know how serious it could be.
"So we're asking people to take steps to protect themselves and others until this picture is clearer."
The UK Health Security Agency is leading investigations centred in the Brackley area of south Northamptonshire into the Omicron cases. A mobile testing unit will be set up at the town's leisure centre from today.
But Mrs Wightman warned some those who have tested positive appear to have travelled outside the local area during the time they were already infectious.
She added: "The aim is to understand whether any spread of the new variant within the local community may have occurred and limit the opportunity for any further infection.
“A mobile testing unit for PCR testing will be available on an ongoing basis from Tuesday at Brackley Leisure Centre to support the local community.
“The unit will test anyone with symptoms as well as anyone who has been identified as a contact of a Covid-19 case, or believes themselves to be.
“Please don’t turn up at the leisure centre without a test appointment. Booking is essential and you can do this online."
Health secretary Sajid Javid told Parliament on Monday (December 6) there are now 261 Omicron variant cases in England.
He added: "This includes cases with no links to international travel so we can conclude there is now community transmission across multiple regions of England."