At a House of Commons public accounts committee (PAC) hearing, Amanda Spielman told MPs that 72 outstanding schools have been inspected this academic year.
Of these, three-quarters have been downgraded and in 30 per cent of cases they have slipped two ratings and are now graded as "requires improvement", with the rest now rated as "good".
Spielman said: "We are increasing this year the number of outstanding we inspect. We are on track to do that.
"We have published reports on 72 so far this academic year.
"Three-quarters of the schools have gone down in their judgement and 30 per cent have gone down not just to good but to requires improvement."
Since 2011, "outstanding" schools are exempt from re-inspection. While concerns raised with Ofsted can trigger an inspection, the system is still leaving some settings without one for as long as 13 years.
As of August 2017, 1,620 schools had not been inspected for six years or more and for 296, the gap was 10 years or more.
In September, committee members raised concerns about so many schools being exempt from inspections for such long periods. They said this was failing to give parents the assurance they need around the quality of their children's education.
Spielman told MPs she shared their concerns. "I don't believe parents like it and I don't believe schools themselves like it very much. They perceive it as introducing unfairness between schools."
She added: "It is a very unsatisfactory position. I'm very uncomfortable with it, but I'm constrained."
She said that of the outstanding schools inspected this year, some were due to concerns over results, while others were due to the length of time since their last inspection, of 12 and 13 years in some cases.
Earlier this month, Ofsted announced plans to overhaul school inspections to focus more on the quality of education over performance data.
Being proposed is to bring forward standard inspections for all schools, including those rated as outstanding.
Currently, schools rated as "good" receive a lighter touch inspection, of just a one-day visit. Ofsted is proposing to increase this to two days to allow inspectors greater scrutiny of standards.
"Off-rolling" - when schools seek to remove challenging or poorly performing students from the school roll through tactics such as encouraging parents to take up home schooling - was also highlighted at the hearing.
Speilman confirmed that, so far, only two schools have been formally identified during an inspection as "off-rolling" pupils.
The inspectorate had flagged 300 schools where this could be an issue.
Spielman said that Ofsted only has the power to report concerns and it was up to school governing bodies, councils, academies and the government to take action.
The two schools where off-rolling has been formally identified are Harrop Fold School in Manchester, which was inspected last October and November and rated as "inadequate" for overall effectiveness after being graded as "good" during its previous inspection.
The other is Shenley Academy in Birmingham, which has also been downgraded from "good" to "inadequate" following Ofsted's visit last October.
"During the inspection, inspectors found evidence to suggest that leaders had 'off-rolled' some pupils in Year 11," states Ofsted's report into Shenley Academy.
"Leaders could not give any valid explanation as to why eight pupils, all of whom were disadvantaged and half had SEND, were removed from the school's roll on the same day in the autumn term 2017."