People with literacy problems should be given more help to complain about public services for the sake of "social justice", the ombudsman has said.
Nick Bennett said it was "feudal" that oral complaints could currently only be handled at his discretion.
People unable to write complaints "shouldn't be dependent on what mood I happen to be in", he told AMs.
However, he said he did not need extra funding to pay for advocates to support less literate people making complaints.
Plaid Cymru AM Simon Thomas has tabled a bill which would boost the ombudsman's powers and extend his scope, including a duty to handle oral complaints.
Giving evidence to the assembly's equalities committee on Thursday, Mr Bennett said: "I find it possibly feudal the way in which the current legislation is drafted.
"I can currently consider an oral complaint should I so consider it appropriate using my discretion.
"Now I'm sorry - people who might have literacy issues have rights too. They shouldn't be dependent upon what mood I or anybody else doing my job happened to be in as to whether or not you're going to exercise that discretion.
The ombudsman stressed he was "not in the business of looking for more complaints" but added: "We always have to challenge ourselves to make sure we're reaching out to the people that need our services the most."
Mr Bennett said his team would spend "hours" transcribing oral complaints and send them back to the complainant "who will hopefully be in contact with an advocate who will be able to advise them that their complaint has been encapsulated sufficiently" before signing and returning the written document.
However, he added: "We only receive 50% of them back.
"Current arrangements are not efficient and we want to make sure that the office - despite the pressures that might exist in terms of ever-increasing numbers [of complaints] - is still doing all that it can to help the most vulnerable that need good quality public services."
Labour AM John Griffiths, who chairs the committee, asked the ombudsman if that meant he wanted to see provision for advocacy support included in the bill.
Mr Bennett replied: "No - we're happy with the provisions of the bill.
"What's critical here is moving away from this issue of discretion.
"We do have a higher incidence of literacy issues than the UK average - therefore, in Wales, this has the potential to be of greater social injustice.
"The issue? It's stigma - some people who might have suffered a real injustice [may] feel that they can only come to my office if they're prepared to write.
"We need to remove that fear factor.
"We're not trying to change the office into some kind of call centre but we do want to make sure that we're sufficiently flexible that we're serving the most disadvantaged."
Plaid Cymru AM Sian Gwenllian asked Mr Bennett if there was a risk he would be creating a "culture of complaining", leading to an unsustainable situation for his office.
In reply, Mr Bennett said a growing culture of complaint was a reality, and that his aim was to ensure "social justice" by allowing everyone to raise complaints effectively.
When the bill was tabled in October, Finance Secretary Mark Drakeford said the Welsh Government would "look positively at measures that will help the ombudsman to undertake his role", but wanted to ensure that public services were not being "over-regulated", as well as considering the cost implications.