Johannesburg is the largest man-made urban forest and Tijuca forest in Rio is the largest urban forest.

https://en.wikipedia.org/wiki/Urban_forest

Okay, by some people's argument it is and since this is not an exact science comparing Tijuca's 3,953 hectares to London's 14,000 even with percentage coverage it would suggest otherwise.

That said lots of cities have a claim: Oslo, Joberg(as you indicate), some places in China.

Is it most trees, densest coverage, or simply largest area that still satisfies the definition of Urban Forest? You can pick a different answer for whichever metric qualifies.