AAIB report here: aaib.dft.gov.uk/cms_resources/G-FORS%203-06.pdf
I too am unfamiliar with 'oscillatory stall' AAIB reports "One other student of the instructor described the oscillatory stall as being in a deep stall with the pilot controlling any wing drop with applications of rudder." Surely the student means 'incipient' stall?
I hated stall practice, one aircraft just wallowed at onset, another did a violent R wing drop. Hammerheads were a 'rush' and spinning was better than any video game.
Initial (RAF) training was in a Chipmunk T10 so the parachute was essential for a 'soft' ride. Was never offered a 'chute for those Cessna armchairs during PPL, could even wear my nylon underwear!
IMO stall ident & recovery should be taught straight after basic flying controls as part of lift, weight, thrust & drag, due to unexpected icing; which could have been a factor in this incident