3) Cover letter ( must state why you need for long treatments otherwise embassy would approve non O visa)
4) Bank Statements ( 3 months at least each month to meet minimum requirements if you show more which is favourable that you have sufficient fund). I used ink stamp by signing brach manager from each bank.
5) Proof of accommodation.
( I booked nearby hospital for one month)
6) Proof of residence of current country ( Tenancy agreement with utility bills)
If you are from visa waving countries, you can do online check-in and print boarding pass and straightaway go to immigration and boarding area if no check baggage.