This corner of the park is called "Johan Sverdrups plass", and also includes the little park across the road.

Sources:
https://www.oslo.kommune.no/natur-kultur-og-fritid/tur-og-friluftsliv/parker-lekeplasser-og-friomrader/hallenparken-johan-sverdrups-plass/

https://no.wikipedia.org/wiki/Johan_Sverdrups_plass_(Oslo)

https://oslobyleksikon.no/index.php/Johan_Sverdrups_plass