
Why did Okiya drink Bourbon, the possible answers are:
Nothing, it is just a coincidence
Okiya is the pyromanic who downed Bourbon for good luck before commiting arson, as mentioned by Conan in the beginning File 638.
Okiya is the Black Organization member named Bourbon due to his deduction skill.
Okiya has the big grudge to the Black Organization member named Bourbon. Okiya is possibility from FBI or CIA.
Okiya is Akai Shuichi in disguise. (Many fans hope so)
Okiya is Vermouth in disguise, that why Ai-chan is afraid to him why she saw Okiya for the first time.
If Okiya drinks Sherry instead of Bourbon, then man fans will expect he is Bourbon who are chasing Ai-chan.
No matter what, we will know in the future.
The story I think:
Maybe the story of Okiya/Bourbon going to be identical to Mega Man 7. Mega Man 7 is the debut of mystery robot named Bass, Bass mentioned to Mega Man that he was battling Dr Wily as well. Later, it revealed that Bass was apparently Dr Wily's creation.
Just like Conan trusts Okiya is a good guy, but later when Conan sees that Kudo residence, Agasa residence or Mouri Detective Agency is kaput and Ran or Ai-chan is kidnapped or serious injured by Okiya. And then Gin reveals to Conan that Okiya's true identify is the spy sends by Black Organisation, in order to investigate that did Kudo Shinichi is still alive and where is Sherry. Gin taunts Conan that he knew that Conan/Shinichi is stupid enough to trust Okiya when Okiya just say a little thing about Holmes and treat Okiya as Holmes' fan and let him stay in Kudo residence.