The bullies from earlier are clearly up to something, but don't want Geron listening to them.
If they didn't know he was listening, they may resume their conversation.
Remember that you can press the spacebar to see interactive elements in your immediate environment, and click the quickhint for a nudge in the right direction.
You should find a hiding spot near the bullies.
Go through the door next to the bullies to eavesdrop on their conversation, and click the signpost to re-read it.
We visited the castle earlier to speak to the chamberlain - there should also be a guard in that area for us to talk to.
It seems the leaf isn't in this guard's trousers or doublet; not the ones he's currently wearing, anyway.
Look around for another uniform - preferably one that isn't being worn.
In the market where we got the plum spirit, a guard's uniform can be seen hanging from the washing line overhead.
It's a little too high for Geron to reach, though, so you'll need to find something that can extend your reach.
Something long and sturdy.
You passed by something that may prove useful earlier; it was indoors.
Return to Geron's home and take the walking cane from the area upstairs.
Use it to knock down the doublet, then right-click on it in your inventory to retrieve the brass oak leaf.
Yes, this leaf wasn't meant for us, after all - Geron needed to collect four copper leaves. Unfortunately, the bullies took one away from us earlier.
It's a classic standoff - you have something they want, and they have something you want.
Talk to the bullies about Olgierd's leaf, and see if you can negotiate.
That's technically all four oak leaves found - we just need to collect the two that Geron found already. See the quickhints for help.
He's hidden them somewhere in his home.
They're sticking out from underneath his pillow on the bottom floor.
Hurry back to the chamberlain and gain an audience with the King!