The journal titled "OLYMPIADS IN INFORMATICS" is published every year, starting with 2007. Only original high-quality scientific papers are part of the journal, and they are all free to access.

You can click on an edition to read it:
      Volume 12 (2018)
      Volume 11 (2017)
      Volume 10 (2016)
      Volume 9 (2015)
      Volume 8 (2014)
      Volume 7 (2013)
      Volume 6 (2012)
      Volume 5 (2011)
      Volume 4 (2010)
      Volume 3 (2009)
      Volume 2 (2008)
      Volume 1 (2007)

A couple of special editions have also been published:
      Volume 11 - Special Edition (2017)
      Volume 10 - Special Edition (2016)

The content links above contain the journal volumes.
Alternatively, you can find an index of all papers as separate files here.