He told a meeting at the party's annual conference in Brighton that Labour had 'saved the NHS since 1997'.
But he added that such gains would be at risk under the Conservatives, citing their plans to make GPs responsible for commissioning budgets.
'It's all very well devolving everything, but you have to ask if (GPs) have the capability and skills to do that,' he said. 'The risk is a lot of GPs will go to the wall.'
Mr O'Brien's comments came at a fringe debate on how the NHS would cope with the end of funding growth from 2011.
He stressed that the NHS was not facing £20 billion of cuts, as has been widely reported, but had to find similar savings 'much of which we want to reinvest in the health service'.
Solutions to the cash crisis aired at the event included a public list of poorly performing NHS managers, and a more outcomes-focused QOF.
Matthew Swindells, health director at consultancy Tribal, said PCTs would need to work with practices. 'GPs are the largest cost pressure in the NHS. Every time they write a prescription or send someone to hospital they spend taxpayer's money,' he said.
He added that cash could be saved by moving hospital care into the community. He said elderly patients were 'incarcerated' because there was nowhere else to treat them. 'It is really expensive to be that rubbish,' he said.