He got up each day at 4am to work on the poems, painstakingly teasing out the symbols and allusions to Japanese culture and classical Chinese literature. The result, Basho's Haiku: Literal Translations for Those who wish to Read the Original Japanese Text, with Grammatical Analysis and Explanatory Notes is a little jewel of scholarship, full of idiosyncratic notes, anecdotes and observations on everything from the 800 varieties of persimmon and the origins of green-tea ice cream to the scientific name for the Japanese cuckoo (cuculus poliocephalus).
And it would never have been written if Mr Oseko had not contracted tuberculosis 40 years ago. He has not had an easy life. Born in 1929, he only just made it through the Second World War: he remembers putting a futon over his head and running outside during one of the firebombing raids on Tokyo, when his own family's house and his aunt's house were burnt to the ground.
Three years after the war ended he was confined to a sanatorium on the outskirts of Tokyo where he was treated for tuberculosis. The disease was then widespread in the war- weary, under-nourished population. An attempt to cure him with surgery nearly killed him. He was to spend eight years in the sanatorium before a new generation of drugs became available and he was healed. But the time was not altogether wasted.
'Apart from some fever, I did not usually feel that sick, so I used to read all the time - anything I could lay my hands on.' The sanatorium had a well-stocked library, and he could order books from outside. He decided to improve his English and to learn French and German as well. He became interested in Beethoven, and in the musician's fight with deafness. 'Fate came and knocked on his door, but he fought back . . . I nearly died after that operation, and Beethoven was a great inspiration.'
From Beethoven to Basho: after being discharged from the sanatorium in 1956 at the age of 27, Mr Oseko had few skills to offer apart from his foreign language ability. He got a job as an English-speaking tour guide, and often found himself accompanying tour groups to parts of Japan that the wandering poet had visited and immortalised in verse 300 years before.
'But I got tired of summarising Basho in five minutes - I kept wanting to give more information, more background. And so I decided to write the book.'
His book does not hold back on the background.
Penetrating the rocks,
The voices of cicadas]
This, one of Basho's better known haiku, prompts a discussion of Japan's 32 varieties of cicada (out of the 1,600 in the world). Mr Oteko then drifts on to the poignancy of the short, 10-day lives of cicadas after they have spent several years underground as larvae, and says this suggests the shortness of life for the poet as well. He links this sentiment to the thinking of a Chinese poet, Li Po, who he points out - way out on a red herring by now, but unrepentant - was used in Gustav Mahler's Das Lied von der Erde.
Mr Oteko also runs us through the different types of capsicum in Japan, the Chinese zodiac animals, the difference between Japanese apricots and Chinese plums, Nichiren Buddhism and the lotus sutra, 800 varieties of persimmon and the Oi river crossing where, for several hundred years, no bridge was built for defensive reasons and travellers were carried across on bearers' shoulders.
Often his notes run to several pages of historical, literary or botanical explanations which, taken together, offer a formidable introduction to Japanese culture. But sometimes even his serendipitous scholarship is brought to an abrupt stop by the poetry itself:
Dropping from a blade of grass,
There flies up again without touching the ground,
To this, Mr Oteko can only add: 'What careful and close observation Basho has made]'