O'Brien was "alarmed by the results" and said that decisive moves were needed, which included taking a close look at whether the Independent and Belfast Telegraph are contributing "anything worthwhile" to the firm.
IN&M's results revealed revenues at the firm's UK division, which includes The Independent, The Independent on Sunday and the Belfast Telegraph, dropped by 14.1% year on year to €115.6m in the six months to June 30.
The firm's total revenues were down 3.7% to €780.4m during the same period.
O'Brien said: "Revenues have declined and the performance of the UK operations, in particular, seems to be in freefall. More worryingly, the state of the balance sheet has deteriorated with net debt increasing in the period by €88m."
There was no comment from IN&M on the new attack from O'Brien.
Sir Anthony O'Reilly, chief executive of IN&M and its largest shareholder, said INM would achieve profits in line with forecasts for the year, "assuming a continuation" of first half advertising trends.