The ombudsman was receiving more than a thousand cases a day in the period and dealt with 161,195 enquiries in total, latest data this week shows.
There was a sharp rise in the number of SIPP complaints it dealt with, FOS figures show.
There were more than 2,800 SIPP complaints in the nine months from April to December last year, an increase of 37% per cent on the previous full year.
Overall, the ombudsman service upheld 33% of all new complaints resolved but the figure for SIPPs was 59%, up on the previous year.
The figures revealed that PPI continued to be the most complained-about financial product, with 40,855 new complaints.
PPI complaints made up nearly 44% of all complaints received by the ombudsman and the uphold rate increased slightly from 26% to 29%.
Caroline Wayman, chief ombudsman and chief executive of FOS, said: “The Financial Ombudsman Service takes on over a thousand new cases every day and our team works hard to get to the bottom of them for consumers and businesses.
“The number of PPI complaints we have taken on recently topped two million, and PPI cases still make up a significant proportion of our work. However, we deal with complaints about the full spectrum of financial products, and we would urge anyone who has a complaint about a financial business to get in touch with us.”
The statistics were published in the latest edition of Ombudsman News.