Was Fairy Tail Inspired by One Piece?
No apprenticeship, no documented debt: the resemblance has a common source.
It is the question every newcomer asks of the art, and it deserves a documented answer rather than a shrug. Mashima was never Eiichiro Oda's assistant, and no statement by either artist records One Piece as an influence on Fairy Tail. What the record does show is two artists of the same generation drinking from the same well.
The resemblance itself is real and was noticed immediately: English-language reviews of the very first volumes in 2008 were already making the comparison. This page separates the three things usually bundled together, the myth, the criticism, and the actual shared influence.
THE EVIDENCE
Influence, apprenticeship, and a shared masterWas he Oda's assistant? No.
The apprenticeship story is the claim to retire first. Mashima's route into the industry is documented and contains no assistant years under anyone: a rookie prize for the one-shot Magician in 1998, won while he worked part-time at an arcade, followed directly by his own serial in 1999.
What the critics actually said
Reviewing the early English volumes, Anime News Network's critics repeatedly noted the resemblance: Carl Kimlinger found the art's similarities to Eiichiro Oda's hard to ignore, while Carlo Santos and IGN's A.E. Sparrow read the same resemblance more kindly. The comparison belongs to the reception of the books, and it was being made by 2008.
The shared source
Both artists stand downstream of the same river. Mashima names Akira Toriyama's Dragon Ball as a favourite of his childhood and one of the works that made him an artist; Toriyama's influence on that whole generation of Magazine and Jump artists is the common cause the two styles share.
Mashima's own north stars
His documented enthusiasms point elsewhere: Dragon Ball and Dragon Quest, Kinnikuman, Miyazaki's films, and, as his stated favourite manga in 2011, Berserk. The guild's inspiration, in his own telling, was not a comic at all but his friends, in bars, being young and finding their callings.