The reaction of metalloxanes [{LM(OH)}(μ-O)] [M = Al (1), Ga (2); L = CH{CMe(NAr)}, Ar = 2,4,6-MeCH, Me = methyl] with an excess of BH·D (D = tetrahydrofuran (THF), SMe) affords annular metallaborate systems achieved through M-O-M cleavage. Compound 1 led exclusively to the formation of eight-membered ring systems [{LAl(μ-O){B(OBu)}(μ-O)}] (3) and [{LAl(μ-O)(BH)(μ-O)}] (6), while for 2 the unprecedented six-membered ring systems [{(LGa)(μ-O)}(μ-O){B(OBu)}] (4) and [(LGa)(μ-O){(BOBu)(μ-O)}] (5) were observed. The use of BH·THF with 1 and 2 led to the concomitant THF ring-opening reaction, while with BH·SMe in THF no such reaction was observed. The metallaborates [LAl{OB(pinacol)}(OH)] (7) and [{LGa(OB(pinacol))}(μ-O)] (8) were synthesized from pinacolborane and the corresponding metalloxane, providing structural evidence that supports the reaction pathways proposed for the formation of 3-6. Density functional theory studies were performed on 3-5 to assess the effect of the metal exchange between aluminum and gallium atoms on the energy of the general ring structures.