Await for list of Tasks

You are looking for Task.WhenAll:

var tasks = ObjectList
    .Where(o => CalculateIfNeedToMakeTaskForO(o))
    .Select(o => OTaskAsync(o))
    .ToArray();
var results = await Task.WhenAll(tasks);
var combinedResults = results.Select(r => "result for O is: " + r);