  The emergence of spatial coherence in a confined two-dimensional Bose gas of
exciton-polaritons with tuneable interactions offers a unique opportunity to
explore the role of interactions in a phase transition in a driven-dissipative
quantum system, where both the phase transition and thermalisation are mediated
by interactions. We investigate, experimentally and numerically, the phase
correlations and steady-state properties of the gas over a wide range of
interaction strengths by varying the photonic/excitonic fraction of the
polaritons and their density. We find that the first order spatial coherence
function exhibits algebraic decay consistent with the
Berezinskii-Kosterlitz-Thouless (BKT) phase transition. Surprisingly, the
exponent of the algebraic decay is inversely proportional to the coherent
density of polaritons, in analogy to equilibrium superfluids above the BKT
transition, but with a different proportionality constant. Our work paves the
way for future investigations of the phenomenon of phase transitions and
superfluidity in a driven-dissipative setting
