  Interaction-driven metal-insulator transitions or Mott transitions
  are widely observed in condensed-matter systems. In multi-orbital
  systems, many-body physics is richer in which an orbital-selective
  metal-insulator transition is an intriguing and unique
  phenomenon. Here we use first-principles calculations to show that a
  magnetic transition (from paramagnetic to long-range magnetically
  ordered) can simultaneously induce an orbital-selective
  insulator-metal transition in rock-salt ordered double perovskite
  oxides $A_2BB'$O$_6$ where $B$ is a non-magnetic ion (Y$^{3+}$ and
  Sc$^{3+}$) and $B'$ a
  magnetic ion with a $d^3$ electronic configuration (Ru$^{5+}$ and
  Os$^{5+}$). The orbital selectivity originates from
  geometrical frustration of a face-centered-cubic lattice on which
  the magnetic ions $B'$ reside. Including realistic structural
  distortions and spin-orbit interaction do
  not affect the transition. The predicted orbital-selective transition
  naturally explains the anomaly observed in the electric resistivity
  of Sr$_2$YRuO$_6$. Implications of other available experimental data
  are also discussed. Our work shows that by exploiting
  geometrical frustration on non-bipartite lattices, novel
  electronic/magnetic/orbital-coupled phase transitions can occur in
  correlated materials that are in the vicinity of metal-insulator phase
  boundary.
