(1) The burden of proving that an injury or occupational disease is compensable and of proving the nature and extent of any disability resulting therefrom is upon the worker. The worker cannot carry the burden of proving that an injury or occupational disease is compensable merely by disproving other possible explanations of how the injury or disease occurred.
(2) Notwithstanding subsection (1) of this section, for the purpose of combined condition injury claims under ORS 656.005 (7)(a)(B) only:
(a) Once the worker establishes an otherwise compensable injury, the employer shall bear the burden of proof to establish the otherwise compensable injury is not, or is no longer, the major contributing cause of the disability of the combined condition or the major contributing cause of the need for treatment of the combined condition.
(b) Notwithstanding ORS 656.804, paragraph (a) of this subsection does not apply to any occupational disease claim.