Thank you for the great list
@zoomaniac ! The sad truth for me is that almost all the cities that you mentioned have a decent zoo in a close proximity. Oslo pick the shortest stick out of them all. The citizens of the only other capital on the list Brussels have access to the two of the best zoos in Europe in short trip by train and are an hour away from the Zoo Giant Pairi Daiza.
AUSTRIA:
Graz -
Turtle Island - probably the Biggest collection of Testudines in the World
BELGIUM;
Brussels -
Plankendael
Gent -
Antwerp
FRANCE:
Marseille -
Parc Zoologique de Barben
Nantes -
Planète Sauvage
Nice -
Jardin Animalier de Monaco
Strasbourg - looks like the Zoo de L'orangerie was closed
GERMANY: -
all the German cities, but Bremen probably are located in a heavy zoo areas.
Bielefeld -
Bonn
Bremen
Essen (Grugapark ?)
Mannheim
Wiesbaden
ITALY:
Bari -
Zoosafari Fasanolandia
Bologna -
Safari Ravenna ?
Catania - not many zoos in Sicilia
Florence -
Giardino Zoologico di Pistoia
Messina - not many zoos in Sicilia
Milan -
Parco Faunistico Le Cornelle is next to one of the Milan's Airports Bergamo, Parco Faunistico La Torbiera, Pombia Safari Park, Oasi di Sant'Alessio
Palermo -
Bioparco di Sicilia Carini
Torino -
Zoom Torino
Venice (excused because of obvious reasons)
Verona -
Parco Natura Viva
-I did not count Genua because of their huge Aquarium with Mammals and Birds, but in fact the city would also fit here)
NETHERLANDS - same as in Germany, Dutch people hardly can complain

Den Haag -
Rotterdam
Groningen -
Utrecht -
DierenPark Amersfoort
NORWAY:
Bergen -
at least Aquarium
Oslo
PORTUGAL:
Porto (Zoo Santo Inacio in Avintes, that does not belong to Porto afaik)
SPAIN:
Zaragoza - only Aquarium
Bilbao -
Cabarceno Park
Malaga (except you count marine parks) -
Bioparc Fuengirola
and a few smaller cities like Oviedo or Gijon
SWEDEN:
Goeteborg -
Nordens Ark, Båros
TURKEY:
Istanbul (Faruk Yalcin Zoo in Darica/Kocaeli, that does not belong to Istanbul afaik)