Aside from the suffixes for plural number and possessive case in substantives, the English dental fricatives s and z are not in regular alternation in native words, the exception being a rare singleton, goose [gu:s] ~ gosling [gózli?], where an s in medial position before the liquid l is pronounced as its lax counterpart z. This position of neutralization encompasses loan words with medial s before r as well, e.g., Israel [ízr?èl].
What is being neutralized here is the distinction between tense and lax obstruents, and the predictable outcome is the unmarked (lax) member of the opposition, namely z. Note that this suspension of distinctiveness applies to non-medial clusters of dental fricative + liquid as well––but with reversed values. Thus in the far more frequent case of s + l in initial position, only the marked s is possible, as in slip, slide, etc. This reversal occurs in marked contexts, which individuates initial position vis-à-vis other positions.
The medial [s] as a variant instead of [z] in the name for the capital of Norway is a spelling pronunciation evidently supported by its foreign provenience, hence exhibiting no alternation between s and z (as in goose ~ gosling). But note withal the sole normative pronunciation of the Norwegian borrowing quisling with a [z], i. e., in conformity with native English phonetics.