During a visit in September Ofsted and health watchdog the Care Quality Commission found that parents in the county face a "struggle" to ensure their children's needs are identified and met.
Inspectors said a particular concern among families was the poor quality and lack of timeliness of education, health and care (EHC) plans, introduced through the Children and Families Act 2014 in order to improve the quality of joint support for children. This meant that when children or young people move school or college, necessary provision was not always in place.
Inspectors found that only a third of plans are completed within the statutory 20-week period and too often they are based on out-of-date information.
"Parents are rightly frustrated by this," a letter outlining the findings states.
"Leaders' work to improve the situation has been ineffectual. The situation has improved a little recently. However, the proportion of plans completed in due time remains low."
Further concerns were raised by inspectors around failures by senior managers to effectively evaluate performance in order to make improvements.
"Self-evaluation is not typically sufficiently detailed to prove fully useful. Too often, leaders do not use performance information well to gain a clear understanding of the impact of their work on improving the effectiveness of services," the letter states.
There is also a lack of strategic direction regarding SEND support, with inspectors raising concerns that no single organisation is taking responsibility for how education, health and social care services work together to support children.
Within schools, inspectors also found a high level of exclusions among children with special educational needs. In addition, efforts in schools to improve provision for pupils with their social, emotional and mental health needs "has not been effective enough", states the inspectors' letter.
Despite the concerns, inspectors found that specialist services, such as those for children with a hearing impairment, are effectively supporting children.
They also found that there were effective safeguarding arrangements in place and that SEND children and young people feel happy and well cared for.
As a result of the findings, Oxfordshire County Council and the county's clinical commissioning group have been told by the inspectorates to submit a written statement on how they plan to improve services.
Hilary Hibbert-Biles, the council's lead member for education, said. "We are determined to deliver these improvements alongside our health partners.
"This is a new national inspection framework and many of the issues highlighted in Oxfordshire are being flagged up elsewhere in the country."
Sula Wiltshire, director of quality and lead nurse at Oxfordshire Clinical Commissioning Group, said: "We acknowledge there are improvements needed and we are working with our partners to ensure these are prioritised so that our most vulnerable children and young people in Oxfordshire have the joined-up services they need and deserve."