Protestors are blocking access to Woodburn Forest near Carrickfergus. One man has been arrested for causing an obstruction.
Contractors turned up on Thursday morning to start preparatory work on the site.
Protestors have maintained a presence there since Monday.
The oil company, Infrastrata, wants to drill an exploratory well on the site. A trailer has been used to block access to it and a number of private cars are also blocking the road outside it.
Campaigners claim that the well could impact the water supply because the site is within the catchment of a reservoir that supplies water to thousands of homes in Belfast and Carrickfergus.
Northern Ireland Water, which has leased the well site to the company, has said it is confident there is no threat to the water supply.
The campaign group Stop the Drill has called for the planning consents to be reviewed by the Department of the Environment.
The company has said that once it starts, it will take seven weeks of drilling to complete the borehole.
In a statement, the company said: "Infrastrata is committed to carrying out this conventional exploration in an environmentally-responsible manner.
"In common with other petroleum exploration sites in the UK, the site at Woodburn Forest has been designed to assure the maximum protection for the environment."
The company says this will involve developing a fully watertight site which will protect local watercourses including the Woodburn River and the nearby reservoir.
All work will be finished within four months.
Police and environmentalists are in a stand off at a County Antrim forest where an oil company plans to drill a well.