One for all was originally all for one's quirk All for one can use multiple quirks , now ...

they do resemble each other, but One for All was NOT originally All for One's quirk. They were brothers both born with similar quirks. All for one simply gave his brother a random stockpile quirk, but the very nature of OfA is that as it gets passed on, it collects the quirks of former wielders.

Actually the younger brother (the one who GOT one for all) was born quirk less. He got one for all from his older brother All for one.

That's not the reason. Lets start here: All for ones brother,the first successor,he was born without a quirk. All for one pitied and looked down on his brother. So he gave him a quirk that stockpiles strength or energy. Its unknown whether all for one did it out of kindess, pity, or maybe just out of spite. AFO's brother then discovers a "useless" quirk: A quirk that passes on quirks. Since he passed on his own quirk, the second successor now has it. The pattern then continues. Although maybe it's just the part of the original person's quirk. Because All Might was still able to use his quirk even after passing it on. So perhaps that's the reason young midoriya can use the previous successor's quirks. Or I could be completely wrong.

That's not the reason. Let's start here: All For One's brother, the first successor, he was born without a quirk. All For One pitied and looked down on his brother. So he gave him a quirk that stockpiles strength or energy. Its unknown whether All For One did it out of kindess, pity, or maybe just out of spite. AFO's brother then discovers a "useless" quirk: A quirk that passes on quirks. Since he passed on his own quirk, the second successor now has it. The pattern then continues. Although maybe it's just the part of the original person's quirk. Because All Might was still able to use his quirk even after passing it on. So perhaps that's the reason young midoriya can use the previous successor's quirks. Or I could be completely wrong. I'm probably wrong. Please don't come for me.

to the OP and Alex:
From Ch. 59 All Might's explanation
"The man had a quirkless little brother. [...] The man forced upon his little brother a quirk that stockpiles power. Yes... He who was thought quirkless, did in fact possess one prior.
Though neither he himself nor anyone around him had ever noticed: the erstwhile meaningless quirk that could only grant itself to another!!
And so the quirk to stockpile power and the quirk that could pass itself on merged together! That is the origin of one for all!"
Yes, AfO had a hand in creating the current One for All (by virtue of providing the stockpile quirk), but the origin of the quirk's aspect of passing on (and possibly the merging) was born from the younger brother himself.

Don't shun me from this one but what if All for One is Deku's dad? I mean you never see the face he had before he fought All Might, we've never seen his dad, his dad's name is a play on hisashiburi which means longtime no see which probably means that he is going to be an essential part of the plot.

I feel like you just one-upped me and I'm sad I worked so hard to right that.

@Aricake, sorry didn't mean to one up you, but I kinda irked me that someone tried to tell me that the younger brother was quirkless or that All for one originally possessed one for all which would be useless on him since his own damn quirk does the same thing basically. I familiar with the origin story of the quirk because I really liked the plot twist and the fact is I rewatch the anime at least once every 3 months (lol). So it's ingrained in my head that the brother was never born quirkless