Widely implemented in Europe over the last three decades, Content and Language Integrated Learning (CLIL) has been researched in some European contexts more extensively than in others, with the Norwegian context being among the least represented. This paper attempts to contribute to filling in the gap by examining the impact of a short-term CLIL intervention on Norwegian ninth graders' oral skills, namely listening comprehension, fluency, vocabulary, pronunciation, and grammar, as well as to contribute to the debates on the selectivity of European CLIL programmes and thus the effect of the latter on generally high European CLIL learners' linguistic achievements. The study investigated the oral development of three groups of five students (high-achievers, mid-achievers, and low-achievers) during the first to the fifth week of the intervention. The main findings revealed that the intervention had exerted: (1) a significant impact on the high-achievers' overall oral development and, specifically, fluency, vocabulary, and pronunciation; (2) a marginally significant impact on the mid-achievers' overall oral development and, particularly, fluency and vocabulary; and (3) no significant impact on the low-achievers' oral skills. The least developed oral skill was grammar. The results hence suggested that, in the given short-term perspective of the intervention, CLIL was mostly beneficial to the high- and mid-achievers.