The attack took place on Tuesday evening at Water Gardens Field at Woughton on the Green.
The Parks Trust is appealing for people to call police if they saw the incident, which happened around 7pm.
And they have warned all dog owners to keep their pets firmly on the lead around livestock.
"Sheep worrying is a criminal offence and as a result of an attack the owner can be fined, incur a criminal record and have the dog confiscated or even destroyed,” said a spokesman,
Witnesses should call police on 101, quoting reference number 2022 0420-0496.
The Parks Trust has around 400 sheep grazing in city fields and parks during spring and summer months.
The sheep help maintain wildlife habitats, creating better conditions for wildflowers, which in turn attract many pollinating insects such as butterflies, bees and hoverflies as well as larger animals and birds.