He confirmed he'll lower the household benefits cap from £26,000 to £20,000 and scrap £3bn a year in grants for the poorest university students.
He's launched a mammoth raid on tax credits - including axing them completely for families who plan to have more than two children.
And housing benefit will be scrapped for those aged 18 to 21, a move the charity Crisis warns will make people homeless.
The announcement was also full of surprises, including a compulsory national living wage for over-25s to hit £9 an hour - but the big promise is already prompting doubts.
Mr Osborne told MPs: "It's a budget that recognises the hard work and sacrifices of the British people over the last five years and says 'we will not put that at risk'.
"It's a Conservative budget that can only be delivered because the British people trusted us to finish the job.
"The British economy is fundamentally stronger than it was five years ago.
"But the greatest mistake this country could make would be to think all our problems are solved. You only have to look at the crisis in Greece to understand."
Acting Labour leader Harriet Harman was quick to dismiss the big promises.
"The Chancellor is said to be liberated without the ties of coalition holding him back," she told MPs. "What we have heard today suggests he has liberated his rhetoric from its connection with reality.
"A Budget for working people? How can he make that claim when he is making working people worse off and scrapping grants for the poorest students.
"One Nation Britain? How can he even stand and say those words when while he cuts tax credits for working people, he has not done enough to stop tax avoidance."
So what are the measures in George Osborne's Budget?
Here's our full rundown:
The Tories will make these £12bn in welfare cuts by 2019:
- Limit child tax credits - worth £2,780 per child - to just two children if the extra children are born after April 2017 (exceptions for twins/triplets)
- Lower the benefits cap from £26,000 to £20,000 (£23,000 in London)
- Force BBC to cover £650m cost of TV licences for the over-75s
- Make social/council tenants who earn more than £30,000 to pay full market rent (£40,000 in London)
- Scrap housing benefit for 18 to 21-year-olds and make them ‘earn or learn’
- Reduce income threshold for tax credits (or similar universal credit) from £6,420 to £3,850
- Scrap tax credits (or similar UC) for non-disabled claimants without children
- Raise rate at which a household's tax credit is reduced as it earns more to 48%
- Reduce tax credit ‘income rise disregard’ - a buffer zone to stop families having their credits clawed back - from £5,000 to £2,500
- Cut the incapacity benefit Employment and Support Allowance by £30 a week for people seeking work, equalling it with jobseekers' allowance
- Freeze working age benefits including tax credits for 4 years - longer than the 2 years previously pledged
Altogether these measures will take tax credit spending to level of 2007/08
George Osborne has also said he will:
Scrap grants for poorest university students - worth up to £3,387 - from April, which would have been worth £3bn a year by 2020
Launch new national living wage for over-25s - starting next April at £7.20 - to reach £9 an hour by 2020
Raise income tax personal allowance from £10,600 to £11,000 in April and raise 40p threshold from £42,385 to £43,000
Raise inheritance tax threshold on wealthy people's homes from £325,000 to £500,000 (double for married couples)
Continue capping public sector pay rises at 1% for 4 years, a move likely to make unions furious
Abolish permanent non dom status for rich who've lived in UK for 15 of last 20 years
Force down social housing rents - but not private rents - by 1% a year for 4 years
Hand the NHS a further £8bn, £10bn more a year in real terms by 2020
Reduce bank levy but introduce 8% surcharge on banking profits
Raise payments for Victoria and George Cross heroes to £10,000 a year for life
Fund terror victims memorial and a specific memorial to people murdered in Tunisia
Lower vehicle tax for 95% of new cars to £140 and create a new roads fund
Freeze fuel duty over the next year
Invest £750m in HMRC to chase tax, raising £7.2bn, and name and shame serial tax avoiders
Devolve fire services, land commission and more children’s services to Manchester as example to other cities
Give devolved areas like Manchester the chance to scrap the 6-hour limit on large shops opening on Sundays
Replace dividend tax credit with a new tax-free allowance of £5,000 on dividend income. Rates of dividend tax to be set at 7.5%, 32.5% and 38.1%
Cut Corporation Tax - already the lowest in the G7 - from 20% to 19% in 2017 and 18% in 2020
Meet NATO pledge to spend 2% of national income on defence every year until 2020