Socio-relational aspects are essential for mental wellbeing (MWB), especially in the oldest old age. Our study aims to explore the socio-relational aspects related to MWB in accordance with the experiences of the oldest old of four European countries; and to examine how these differ between Mediterranean and Nordic people. A total of 117 participants aged 80+ years old were recruited, and 23 focus groups were performed. Qualitative content analysis identified five main themes. Family seemed to be the most important driver of the MWB of the oldest old, followed by relationships with close friends. Participants felt better when they had a sense of being needed, cared for, and connected. Loneliness and isolation negatively affected MWB, although solitude was appreciated. Differences appeared between Mediterranean and Nordic regions. Initiatives to promote positive interactions with family and friends, as well as social activities within the community, may contribute to strengthening MWB in the oldest old.