Dynamic where clause (OR) in Linq to Entities

In the post here I learned how to build a dynamic query using the deferred execution of Linq. But the query is actually using an AND concatenation of the WHERE condition.

How can I achieve the same query but with an OR logic?

Due to the Flags enum, the query should search for Username, WindowsUsername or both:

public User GetUser(IdentifierType type, string identifier)
{
    using (var context = contextFactory.Invoke())
    {
        var query = from u in context.Users select u;

        if (type.HasFlag(IdentifierType.Username))
            query = query.Where(u => u.Username == identifier);

        if (type.HasFlag(IdentifierType.Windows))
            query = query.Where(u => u.WindowsUsername == identifier);

        return query.FirstOrDefault();
    }
}