"ÖS" is short for "Öppen samling", which is the Swedish name for the open collection at that library. Books in the open collection has the call number "ÖS XX". If the book is available, you can collect it yourself from the shelf (but don´t forget to check it out)!
The letters after ÖS indicates what shelf it is located at, therefore you need to remember or take note of these letters. A book in ÖS cannot be requested in advance, unless you want to pick it up at another of the Lund University libraries.