I found out the right Oring to head returning technique.
There is upper and lower section in the head. Lower section is tighter, upper section is wider… Or how to say head has transition from narrow to wide section when we watch into head internal diameter and narrow section uses mentioned oring while wider section uses brass pill oring so that way we have double oring sliding mechanism (almost no movments at all if you placed O rings properly…

So what you need to do? You need to place neck section of flashlight into head but watch very carfuly untill it starts transition from narrow to wider section… When it protrudes few mm from tighter to wider section you got to put mentioned oring inside with a help of a toothpick or similar tool… Now in this point when you have inserted oring you gotta make tool out of A4 printer paper; tube that will embrace neck section OD and with careful and gently press it will will be pushing oring to bottom narrow section. You gotta to have feeling for that because at same time you must do zoom out action while pressing with paper tube.

For returning of oring some kind of ultra thin metal sheet tube could also do the trick as a more permanent tool than paper for someone who needs to mod lets say 20 –100 of this lights.