[en] The polarization of positrons in the decay of ¹⁴O (pure Fermi) has been measured relative to that of ¹⁰C (pure Gamow-Teller) using a new technique based on the time-resolved spectroscopy of hyperfine positronium decay. Combined with the absolute experimental value for Gamow-Teller decay polarization, the result is P(¹⁴O) = (1.02±0.056). This constitutes the first use of this technique in an online configuration, and demonstrates its ability for reaching eventual precisions of 0.1% in relative polarization measurements. (orig.)