Some of them GYW, some of them HW...
I believe the most formal black oxford will be the seam across the captoe. Brogueing or medallion will depend on the environment you works for. The general rule applies. The more decoration there are, the less formal it will be...
Well color choice for chukka depend on your general pairing as well... Color choice is up to individual. Navy is a nice color also
Actually the black oxford is my first pair of GYW and being an inexperienced shoe nerd when first started, I opt to go for more decoration since I am paying for the same price...
Also depend on how the pairing is for what occasion. Although black pair is meant for boardroom meeting, this is not happening to me as I am not in C level yet hehe
I usually wear my black captoe with medallion for interview which I believe is kinda acceptable. And my working environment don't warrant for a strict dress code for shoe unlike in law industries I believe...