The present invention relates to a tubular oil separator providing separation of respective fluid components mixed in fluids from oil wells, wherein the tubular oil separator is arranged to mitigate problems related to turbulence and possible non-Newtonian fluid behaviors of fluid components mixed in the fluids from the oil wells in the oil separator. The invention further relates to a method of operating a separator. Moreover the invention relates to a system of multiple separators. Inventive aspects of the separator comprises an elongated outer, closed tubular section and an elongated, inner tubular section, which is closed in one end and open in another end, - where the inner tubular section is arranged inside the outer tubular section, - and where oil well substances are introduced into the open end of the inner tubular section via a tube feed section passing through the outer tubular section and into the inner tubular section, - and where the inner tubular section comprises multiple, elongated and parallel slots arranged in a longitudinal direction of the inner tubular section in a circumferential manner, - where the inclination of the separator facilitates separation of the oil well substances into lower density substances and higher density substances, - where the lower density substances by buoyancy drift upward through the slots and exit via an upper outlet in the outer tubular section and higher density substances sink downward through the slots and by gravitation exit via a lower outlet in the outer tubular section.