If BSkyB is found guilty of abusing its dominant position in the market, it could face fines of tens of millions of pounds or be broken up.
The OFT said that the inquiry, which had been scheduled to close this month, had taken longer than expected "because it is such a complex case".
There is some speculation, however, that the delay could be linked to a meeting chancellor Gordon Brown had with Rupert Murdoch, chairman of BSkyB parent News Corp, on Wednesday.
Although the OFT is independent of the government and there has been no suggestion of political interference, it is believed that the government would be hesitant to upset media owners ahead of a referendum on the euro.
Murdoch's News International publishes The Sun and The Times, which could lend vital support to the government's position on the euro in the event of a referendum.
Another possible reason for the delay, according to analysts, is that the OFT could be taking time to secure undertakings from BSkyB about future practice.
ONdigital filed the complaint in December, accusing BSkyB of charging unfair prices for subscription to its channels, including sport and film offerings. It also complained that BSkyB's deals with some digital services prevented ONdigital getting access to the channels. The inquiry had been expected to last six months.
Meanwhile, the OFT has ordered a new round of consultation into the BBC's £300m proposal to launch four new digital channels.
The move will be seen as a snub to BBC director general Greg Dyke, who this week asked for a swift ruling from the new culture secretary Tessa Jowell.
The ruling will scupper the BBC's plans to launch some of the services by the end of the year, as a decision on whether to approve the licence-fee funded channels will not be made until autumn.