A child was pulled off his bike and repeatedly punched in the head by two boys of a similar age.
The assault on the 14-year-old occurred next to Banbury train station last Monday (July 3) at around 3.30pm.
Police are now trying to locate a 'Good Samaritan' who helped the child after he was attacked.
READ MORE: Boy, 13, arrested after child assaulted
A spokesperson for Thames Valley Police said: "A woman very kindly stopped and helped the young victim, however, we do not know who she was.
"We would really like to get in touch with her, to thank her for her actions and speak with her about the incident.
"If this was you, or you believe you may have witnessed any of the incident, please call 101 and quote reference number 43230294397."
A 13-year-old boy has been arrested on suspicion of assault.
The investigation is linked to the Banbury Neighbourhood Team response to the group of youths responsible for ongoing anti-social behaviour and offending which has been causing concern to the community.
Last week, the force marked Anti-Social Behaviour Awareness Week.
On July 6, a spokesman for Thames Valley Police said: "We currently have eight youths subject to either police or court bail conditions as we continue to tackle this group who have been causing concern to the community.
"We will be actively checking these conditions are adhered to and working with our partners to curb this offending."
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