This study investigates cross-linguistic transfer in the production of long mid front vowels [øː] and [œː] by simultaneous bilingual Finnish and Finland-Swedish speakers in Finland. In Swedish, the phoneme /ø/ can be realised as the allophones [ø] and [œ], while in Finnish, only [ø] is used. Combining approaches from sociophonetic and bilingual transfer research, the study used acoustic analysis to compare the height and fronting of [øː] and [œː] produced by bilingual and monolingual Finland-Swedish speakers in three different speech styles on a continuum of formality. The data from 115 participants are stratified according to language background, speech style, region, and age. The statistical analysis indicates increased overlap of [øː] and [œː] in the vowel spaces of bilingual speakers, particularly in informal speech. The results suggest a potential effect of Finnish transfer on the distinction of the phonetic variants in simultaneous Finland-Swedish bilinguals, as well as demonstrate the importance of considering speech style in bilingual transfer research.