Nutter's plan, the Actual Value Initiative (AVI), was designed to correct decades of inaccurate and inequitable assessment by taxing property based on actual market values.
Under AVI, many lower-income homeowners would see a small tax break. But some affluent homeowners, especially those in developing neighborhoods, could expect the kind of huge tax increases O'Brien described.
O'Brien represents Northern Liberties and Queen Village, neighborhoods that could experience some of the worst AVI sticker shock.
Nutter's spokesman, Mark McDonald, said O'Brien was wrong in his characterization of the bill, which authorizes the city to lower the tax rate as it shifts to AVI.
"There are a lot of people in the city who have been paying too much," he said. "Under AVI, for the first time, we're going to have an accurate assessment."
Because properties are now assessed well below market value, the current tax rate is artificially high. The rate would have to be lowered significantly under an actual value system to collect the same amount of revenue.
But without the bill passed Wednesday, the city would not have been able to do so because of a rule meant to protect school funding.
O'Brien - like many of his counterparts on City Council - also expressed frustration that the Nutter administration has yet to share the results of a citywide reassessment that would determine the tax rate and what property owners would pay in taxes.
Council members balked at implementing AVI last spring, saying they could not predict all the consequences of AVI - and attempt to alleviate the negative ones - without having that data.
"We only have half the equation here," O'Brien said. "They can't provide the basic data that's needed."
The administration now says the numbers will be available in December, and owners should find out in February what value the city has attached to their properties.
Nutter, Council President Darrell L. Clarke, and School Reform Commission Chairman Pedro Ramos all had urged Philadelphia's House delegation to pass the bill before Wednesday, the final day of voting before lawmakers recessed for the year. They did not want to have to try again next year with a new General Assembly, possibly with new leadership.
After O'Brien threatened to scuttle the legislation in the House, the language was amended into a Senate bill, which passed Wednesday afternoon. The bill then needed only House concurrence.
Contact Troy Graham at 215-854-2730 or email@example.com, or follow on Twitter @troyjgraham.