The watchdog branded the county council's children's services "inadequate" after a June inspection.
It said 261 children who needed help in the county had not been allocated a social worker, despite many living in "neglectful and unsafe circumstances".
The authority pledged it would turn things around "as quickly as we can".
In its report, Ofsted said the county's most vulnerable children were not being protected or given appropriate help when needed.
It said key areas of social work had "fundamental weaknesses" and it took the authority too long for social workers to see children most at risk.
The watchdog blamed some problems on the recent re-organisation of the department and its focus on putting new systems in place, which in turn failed to spot whether the basic requirements of social work practice were being met.
Ofsted added that poor leadership, social worker supervision and record-keeping had contributed to the authority's low grading.
It said "immediate action" must take place to ensure vulnerable children and their families have adequate access to support, with the roles of the authority's leadership also needing to be clearly established.
Angela Macpherson, Conservative cabinet member for children's services, said: "We must get back to a 'good' rating or better as soon as we can.
"We are already finalising a new action plan in response to today's report so it's crystal clear to everyone how and when these improvements will be delivered."
She pledged that the council "will turn this around as quickly as we can".
Children in Buckinghamshire are being put at risk by "widespread and serious" failings by the county's safeguarding services, according to Ofsted.