Hello! I am trying to upgrade my bike’s Octalink BB 118 mm to a 105 5600 BB and crankset. The problem is I can not make the plastic pin of 105 left leg fit the hole. I think the axle is too short or BB is too wide? Thanks in advance.!
Check your shell width. Shimano road cranks only work with 68mm shells and require 'road' version of the Hollowtech BB. (MTB cranks / MTB BBs work with 68 and 73mm shells.) I think it's likely you have a 73mm shell.
See this answer for details on the difference between road and MTB BBs.
Update: just looked at the pic of your old BB and it is in fact marked '73'.