[en] Oscillations of B⁰_d and B⁰_s mesons have been studied at LEP with the Delphi detector. In the Standard Model, the oscillation frequencies are linked with CP violation parameters, and the interest of their determination is presented in the first part. To reach a direct measurement of these oscillation frequencies, the B proper time is reconstructed on an event-by-event basis. After a description of the Delphi detector, the event selection and the algorithms used to reconstruct their geometrical and energetic characteristics are presented. The energy reconstruction allows to measure the b quark fragmentation function. Then, using one million hadronic Z⁰ decays, a measurement of the B⁰_d oscillation frequency is derived from the selection of 240 events containing in opposite hemispheres two high p_t same sign leptons, one of them being accompanied by a D meson partially reconstructed in three charged tracks. Finally, a combined study of kaon-lepton correlations and dilepton events shows which limit could be set in the near future on the value of the B⁰_s oscillation frequency