Is ONE PIECE episode 310 filler?
Canon No, episode 310 of ONE PIECE is canon.
#310
From the Sea, a Friend Arrives! The Straw Hats Share the Strongest Bond!
友、海より来る！麦わら一味最強の絆
Aired 20 May 2007
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.