A flock of Soay-Hebridean cross black sheep were introduced to the site last year as part of a variety of landscape initiatives at the 40-acre beauty spot.
Volunteers have been working closely with English Heritage for more than three years on plans to boost the natural environment at the site.
The sheep have been thriving, helping to manage bracken overgrowth in a way which will enable native flora and fauna to flourish and protect the underlying archaeology. However, there have been numerous incidents of sheep being bothered by dogs at the site over in recent months, culminating in three animals being killed last month.
Helen Allen of English Heritage said: “Since the sheep were introduced to the hillfort last year, they have been doing fine work. They have not only helped us to keep the bracken at bay, but also been proving very popular with local residents who enjoy walking on the site.
“This is why it is particularly sad that these three sheep have been killed as a result of irresponsible dog owners who have not kept their pets under control. We will shortly be installing new gates with signage on site to try to remind people, but we need dog owners to play their part by keeping dogs under control at all times, and always on the lead in areas where sheep are grazing.”
Tim Malim of Oswestry Heritage Gateway volunteers said: “It is tragic that these three sheep have been killed and all dog owners must do their bit to protect those that remain. Though the sheep may not always be visible at first glance as they enjoy hiding in the ramparts and undergrowth, it can only take a dog seconds to smell them and know where they are."
Hillfort users are being asked to play their part by keeping dogs on the lead at all times when walking across the plateau at the top of the site and also when accessing or leaving the site at the western entrance which falls within the sheep grazing area.
Native to the British Isles, the Soay-Hebridean cross-breed chosen is hardy and thrives well on rough grassland and heath, making it popular for conservation grazing.
"The distinctive, horned breed is sometimes referred to as the Iron Age sheep, as it was probably the type farmed by our ancestors on hillforts like Old Oswestry and within the surrounding landscape," Tim said.