Hi Longhill,
It mostly depends on how long notice you need to quit your current job as well as course availability.
I did my medicals on 23 Sep, ought to have entered 25 Nov OBS but due to some miscommunications about my notice period, I ended up with the 6 Jan OBS. One of the fellas who did medicals with me started his OBS on 13 Oct as he was unemployed at that point in time.