Reactions of 5,6‐dihydro‐4H‐1,2‐oxazine N‐oxides with enol diazoacetates were studied. A particular reaction path depends on the amount of catalyst and the order of the addition of the substrates. Use of Rh2(Oct)4 (2 mol.%) leads to chemoselective [3+3]‐annulation producing 1,2‐oxazine‐fused 1,2‐oxazine derivatives. With lower catalyst loadings (0.03 mol.%) enol diazoacetates are converted to cyclopropene derivatives, which in situ react with 1,2‐oxazine N‐oxides via tandem [3+2]‐cycloaddition‐rearrangement producing oxazine‐fused aziridines. Both transformations showed a wide substrate scope and produced target products with good yields and diastereoselectivity, thus allowing selective preparation of these rare heterocyclic systems. A mechanistic rationale for observed chemo‐ and stereo‐ selectivities was proposed.