string location = "Oslo";
var training = (from item in doc.Descendants("item")
                         where item.Value.Contains(location)

                      select  new
                         {
                             event = item.Element("event").Value,
                             event_location = item.Element("location").Value
                         }).ToList();

<training>
   <item>
      <event>C# Training</event>
    	<location>Prague</location>
    	<location>Oslo</location>
    	<location>Amsterdam</location>
    	<location>Athens</location>
    	<location>Dublin</location>
    	<location>Helsinki</location>
   </item>
   <item>
        <event>LINQ Training</event>
    	<location>Bucharest</location>
    	<location>Oslo</location>
    	<location>Amsterdam</location>
    	<location>Helsinki</location>
    	<location>Brussels</location>
    	<location>Dublin</location>
   </item>
</training>

string location = "Oslo";
var training = from loc in doc.Descendants("location")
               where loc.Value == location
               select new
               {
                   event = loc.Parent.Element("event").Value,
                   event_location = loc.Value
               };

string location = "Oslo";
var training = from loc in doc.Descendants("location")
               where loc.Value == location
               select new
               {
                   event = loc.Parent.Element("event").Value,
                   event_locations = loc.Parent.Elements("location")
                                               .Select(e => e.Value)
               };

for (var entry in training)
{
     Console.WriteLine("Event: {0}; Locations: {1}",
                       entry.event,
                       string.Join(", ", entry.event_locations.ToArray());
}

var training = (from item in root.Descendants("item")
                where item.Value.Contains(location)
                select new
                {
                    name = item.Element("event").Value,
                    location = (from node in item.Descendants("location")
                               where node.Value.Equals(location)
                               select node.Value).FirstOrDefault(),
                }).ToList();