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.

David was one of 60 cuckoos fitted with trackers by the British Trust for Ornithology (BTO) from 2011 and has made return trips from Africa since.
But his tracker fell silent a few months ago.
A team on BBC Radio Wales' Country Focus is now trying to lure David into the Tregaron woods using a fake female.
Presenter Gilbert John said: "We're hoping to find out whether the lack of a signal for David shows that he is dead, or simply his equipment [is dead]."
The BTO tracking project was set up to discover why half the UK's cuckoos have been lost in the last 20 years.
Since 2011, many of the birds have died or their transmitters have failed, with David one of the remaining six that make  the migration from the Congo rainforest, where they winter, to Wales.
In previous years, he has returned to the UK between 28 April and the 14 May and experts expected him to be on the move from his wintering location, if not back on his breeding grounds, by now.
The BTO has set up a mist net, a plastic cuckoo and the accompanying recording of a female in woods in Tregaron.
"The hope is to entice David if he's within earshot. [Although], I have to admit, she's not a very attractive cuckoo," said John.
Kelvin Jones, Welsh development officer at the British Trust for Ornithology, said a lot of people think the newer satellite technology would eliminate the need for the more traditional "ringing" system.
"But without the ringing system - that is catching the birds and handling the birds - we cannot use the new technology," he said.
Mr Jones said the tracking project had started to unravel the mystery of cuckoo movements.

Hywel Dda University Health Board is the first in Wales to launch the "Push the Button" system to curb smoking at Carmarthenshire, Ceredigion and Pembrokeshire hospitals.
It enables people to anonymously push a red button which triggers an announcement reminding them hospitals are smoke-free.
The policy also includes e-cigarettes.
The bilingual messages were recorded by pupils at Pembrokeshire's Wolfscastle school and the buttons can be found near the main entrances to Bronglais, Glangwili, Prince Philip and Withybush hospitals.
Teresa Owen from the health board said: "While the majority of staff, patients and visitors to our sites respect this policy, some people continue to smoke on-site.
"Everyone has the right to breathe fresh air, especially when visiting a health care facility and we regularly receive complaints about people smoking on our sites."
Police and environmentalists are in a stand off at a County Antrim forest where an oil company plans to drill a well.
[NEXT_CONCEPT]
Conservationists are searching for a missing cuckoo called David who usually returns to his Ceredigion breeding grounds.
[NEXT_CONCEPT]
A PA system is being launched at hospitals in mid and west Wales to discourage smokers from lighting up.