Were you brave enought to ask him if he liked OP? Sometimes when I see someone with OP references I say to them "Good T-shirt", "Good cap" or sth along those lines.
BTW, this is the T-shirt and cap I usually wear:
Cap (bought from 399animeshop)
Dry-fit T-shirt (I vectorized the design myself, copied from another sources I don't remember currently, likely google images)
I also own a key-chain
a mug
and a Luffy wanted poster (post timeskip, pre dressrosa arc)
The last three I bought them when I went to Mugiwara Store in Shibuya Parco in 2014 (Tokyo)