The Licensed Taxi Drivers Association (LTDA) parked a billboard next to Northcliffe House, Kensington, with a mock-up of Osborne's face below a Standard masthead. The headline read: "Lowering London's standards".
The organisation has accused the former Chancellor of putting pressure on ex-Mayor of London Boris Johnson to drop proposed Transport for London (TfL) regulations on drivers of private-hire-app Uber.
"[Freedom of Information Requests] show that George Osborne behaved like a paid-up lobbyist for Uber, abusing his position to stop the introduction of much needed regulations on public safety," said Steve McNamara, the general secretary of the LTDA.
"He's now working for BlackRock, a firm that has invested millions of pounds in Uber – a £50bn company that paid just £400,000 in tax in the UK last year.
"This lack of transparency means taxi drivers fear for the Evening Standard's future as the impartial voice of London with George Osborne as editor."
In 2016 TfL dropped the plans, including making Uber customers wait five minutes between booking their car and starting their journey, after more than 200,000 people signed a petition.
But the Information Commission watchdog is probing Number 10 and TfL over emails sent to London Assembly Transport Committee chair Caroline Pidgeon after she claimed the responses were "completely different".
Osborne, meanwhile, told the Evening Standard newsroom that he was "very excited" to be taking up the post, despite having no prior journalism experience.
"It's a really important time in our country when people are going to want the straight facts, the informed analysis so they can make the really big decisions about this country's future," he said.
"The Evening Standard is going to provide that and it is going to entertain along the way. Now I've got to get in there – we've got a paper to get off stone so I better get started."
Osborne will not seek re-election as the MP for the Cheshire seat of Tatton at the general election on 8 June.