Within the recently proposed new approach we investigate the problem of neutrino oscillations in a magnetic field. Probabilities of relativistic neutrino flavour $\nu_e^L \leftrightarrow \nu_{\mu}^L$, spin $\nu_e^L \leftrightarrow \nu_{e}^R$ and spin-flavour $\nu_e^L \leftrightarrow \nu_{\mu}^R$ oscillations are calculated on the base of the exact neutrino stationary states in the magnetic field accounting for the whole set of possible conversions between four neutrino states. It is shown that there is an
entanglement between neutrino flavour and spin oscillations and in the general case it is not possible
to consider these two types of neutrino oscillations separately. In particular, the probability of flavour oscillations exhibits quite complicated dependence on vacuum and magnetic frequencies.