Organosiloxane block copolymers, curable compositions, and solid compositions derived from these block copolymers are disclosed. The organosiloxane block copolymers comprise: 40 to 90 mole percent disiloxy units of the formula [R12—SiO2/2] 10 to 60 mole percent trisiloxy units of the formula [R2SiO3/2] 0.5 to 35 mole percent silanol groups [≡SiOH] where R1 is independently a C1 to C30 hydrocarbyl, R2 is independently a C1 to C20 hydrocarbyl, wherein; the disiloxy units [R12SiO2/2] are arranged in linear blocks having an average of from 10 to 400 disiloxy units [R12SiO2/2] per linear block, the trisiloxy units [R2SiO3/2] are arranged in non-linear blocks having a molecular weight of at least 500 g/mol, and at least 30% of the non-linear blocks are crosslinked with each other, each linear block is linked to at least one non-linear block, and the organosiloxane block copolymer has a molecular weight (Mw) of at least 20,000 g/mole.