Alassane Plea's first-half strike handed Nice their first ever European away victory as they secured a 1-0 Europa League win over Salzburg.
Standard Liège came back from 0-2 down to draw 2-2 with Panathinaikos, in Group G.
Southampton's Virgil van Dijk said Southampton should have put the game away easily in the first half.
Celta Vigo and Ajax shared four goals in an entertaining 2-2 draw to keep themselves at the head of Europa League Group G.
Southampton wasted the chance to take a significant step towards the Europa League's knockout stages with a disappointing 1-0 defeat at Inter Milan.
Alexandre Pato saved thorough embarrassment at the hands of Osmanlispor by scoring in the 74th minute.