The development of quantum mechanics since the beginning of the 20th century has guided the exploration of nature at the atomic scale. The analysis of neutral kaons in the middle of the century highlighted the quantum mechanical behaviour of subatomic particles coming from cosmic rays. The advances in particle accelerators allowed to produce abundant samples of heavy mesons and gain insights into the fundamental quantum fields governing the interactions of matter. The measurement of wave-like properties, such as oscillation frequencies and interference patterns, is achieved by studying the rate of change of neutral particles into their anti-matter counterpart. The data used to study particle-antiparticle oscillations are collected by specialised experiments looking for the decays of heavy quarks. The objective of this thesis is the measurement of the oscillation frequency of the B_s meson, a neutral state made by the combination of a beauty and a strange quark. The associated anti-meson state, Bbar_s, is distinguished only by its charge-conjugated quark content, referred to as flavour. The physical states with definite mass and lifetime, B_H and B_L, also called the heavy and light mass eigenstates, are therefore a superposition of the flavour eigenstates. The experiment can select a flavour eigenstate by choosing a specific final state into which only a B_s or Bbar_s meson can decay into. Comparing flavour eigenstates at the time of production and decay, it is possible to probe their time-dependent transition probability, depending on the mass difference between the B_H and B_L states. The B_s-Bbar_s system oscillates at the astonishing rate of about fifty billion times per second between the two flavour eigenstates due to this tiny mass difference. The oscillation frequency is related to the mass difference as nu = delta_m c^2 / hbar, as a consequence of the wave equation and according to the principle of particle-wave duality of quantum mechanics. The measured frequency corresponds to a mass difference of one part in a hundred of an electron-volt (eV/c^2). It is therefore not possible to directly probe the mass splitting of the B_H and B_L states with the current experimental techniques, which can determine the mass of the B_s meson with a resolution that is about a billion times larger, around tens of mega electron-volts (MeV/c^2=10^6 eV/c^2). However, by exploiting the large relativistic boost of the particles produced in high-energy proton collisions, with a centre-of-mass energy at the tera electron-volts (TeV=10^12 eV) scale, it becomes feasible to resolve the fast B_s-Bbar_s oscillations with a sub-millimetre decay-length resolution and study the effect of the mass splitting in the decay time spectrum. The measurement of B_s-Bbar_s oscillations does not only offer a laboratory to study the classical quantum mechanical properties of the subatomic world, but also provides a gateway to the relativistic quantum fields constituting all elementary particles and fundamental interactions. Since boson and fermion fields enter virtually in B_s-Bbar_s transitions, by sharpening the experimental knowledge on the oscillation frequency one can gain insight into the constituents of matter and their interactions. This thesis reports the most precise measurement to date of the B_s oscillation frequency delta-m_s, with a relative uncertainty below one part in a thousand. The B_s oscillations are studied using data from the LHCb experiment, collected during Run2 of the Large Hadron Collider (LHC) at CERN. The B_s mesons are reconstructed using the “flavour specific” B_s->D_sPi decay, for which the identity of the B_s at decay can be inferred from the charge of the Pion in the final state. The outcome of this research is documented in: “Precise determination of the B_s-Bbar_s oscillation frequency” (LHCb Collaboration), which is accepted for publication in Nature Physics.