The 24-year-old moves to Roots Hall on a free transfer after leaving the Clarets by mutual consent.
O'Neill had been on loan with Southend before making his stay a permanent one.
Phil Brown's side return to League One action on September 5 when they host Peterborough United.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | Manchester CityMan City | 27 | 21 | 3 | 3 | 64 | 17 | 47 | 66 |
| 2 | Liverpool | 26 | 18 | 6 | 2 | 70 | 20 | 50 | 60 |
| 3 | Chelsea | 25 | 14 | 8 | 3 | 49 | 18 | 31 | 50 |
| 4 | Manchester UnitedMan Utd | 27 | 13 | 8 | 6 | 44 | 34 | 10 | 47 |
| 5 | West Ham UnitedWest Ham | 27 | 13 | 6 | 8 | 46 | 34 | 12 | 45 |
| 6 | Arsenal | 24 | 14 | 3 | 7 | 38 | 27 | 11 | 45 |
| 7 | Tottenham HotspurSpurs | 25 | 13 | 3 | 9 | 35 | 32 | 3 | 42 |
| 8 | Wolverhampton WanderersWolves | 26 | 12 | 4 | 10 | 24 | 21 | 3 | 40 |
| 9 | Southampton | 26 | 8 | 11 | 7 | 34 | 37 | -3 | 35 |
| 10 | Brighton & Hove AlbionBrighton | 26 | 7 | 12 | 7 | 25 | 30 | -5 | 33 |
| 11 | Crystal Palace | 27 | 6 | 12 | 9 | 37 | 38 | -1 | 30 |
| 12 | Aston Villa | 25 | 9 | 3 | 13 | 33 | 37 | -4 | 30 |
| 13 | Leicester CityLeicester | 23 | 7 | 6 | 10 | 37 | 43 | -6 | 27 |
| 14 | Newcastle UnitedNewcastle | 25 | 5 | 10 | 10 | 28 | 45 | -17 | 25 |
| 15 | Brentford | 27 | 6 | 6 | 15 | 27 | 44 | -17 | 24 |
| 16 | Leeds UnitedLeeds | 26 | 5 | 8 | 13 | 29 | 60 | -31 | 23 |
| 17 | Everton | 24 | 6 | 4 | 14 | 28 | 41 | -13 | 22 |
| 18 | Burnley | 24 | 3 | 12 | 9 | 22 | 30 | -8 | 21 |
| 19 | Watford | 26 | 5 | 4 | 17 | 25 | 47 | -22 | 19 |
| 20 | Norwich CityNorwich | 26 | 4 | 5 | 17 | 15 | 55 | -40 | 17 |
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