Reaction of [(PMe 2 Ph) 2 PtB 8 H 1 2 ] with KH and [{(η 5 -C 5 Me 5 )RhCl 2 } 2 ] followed by chromatographic separation in air unexpectedly gave the ten-vertex oxametallaborane [9,9-(PMe 2 Ph) 2 -arachno-9,6-PtOB 8 H 1 0 ] (1, 11%), which has a classical arachno ten-vertex geometry with the {O} and{Pt(PMe 2 Ph) 2 } vertices in the prow (6)- and (9)-positions; the oxygen atom is therefore bound contiguously to boron atoms only. Reaction of [PtMe 2 (PMe 2 Ph) 2 ] with [6,9-(PMe 2 Ph) 2 B 1 0 H 1 2 ] generates its ten-vertex metallaborane cognate [6,6-(PMe 2 Ph) 2 -arachno-6-PtB 9 H 1 1 -9-(PMe 2 Ph)] (3, 10%), and completes a coherent B->C->N->O cluster constituent sequence. Compounds 1 and 3 are characterised by single-crystal X-ray diffraction analysis and NMR spectroscopy.