List<string> customersTitles = Odata.OrdersService.Select(o=>o.CustomerTitle).Distinct().ToList();

List<Customer> customers = Odata.CustomerService.Where(m => customersTitles .Contains(m.CustomerTitle))

public class CustomerController : EntitySetController<Customer, int>
{

    [Queryable]
    public override IQueryable<Customer> Get()
    {
        Expression filter = this.QueryOptions.Filter.ToExpression<Customer>();
        return db.Query<Customer>(filter as Expression<Func<Customer, bool>>);
    }
}

List<Customer> customers = Odata.CustomerService.Where(m => customersTitles .Contains(m.CustomerTitle))

List<Customer> customers = Odata.CustomerService.Where(m => m.CustomerTitle == customerTitles[0] || m.CustomerTitle == customerTitles[1]); // and so on

var titleFilterList = customerTitles.Select(title => String.Format("(CustomerTitle eq {0})", title));
var titleFilter = String.Join(" or ", titleFilterList);
var customers = Odata.CustomerService.AddQueryOption("$filter", titleFilter).Execute().ToList(); // you may have to cast this.

public static IQueryable<T> WherePropertyIsIn<T, TSet>(
    this IQueryable<T> query,
    IEnumerable<TSet> set,
    Expression<Func<T, TSet>> propertyExpression
) {
    var filterPredicate = set.Select(value => Expression.Equal(propertyExpression.Body, Expression.Constant(value)))
        .Aggregate<Expression, Expression>(Expression.Constant(false), Expression.Or);

    var filterLambdaExpression = Expression.Lambda<Func<T, bool>>(filterPredicate, propertyExpression.Parameters.Single());

    return query.Where(filterLambdaExpression);
}

var allowed_states = getAllowedStates();
var maxPopulation = getMaxPopulation();

// Instead of...
var cities = context.Cities.Where(c => allowed_states.Contains(c.State) && c.Population <= maxPopulation);

// Use...
var cities = context.Cities.Where(c => c.Population <= maxPopulation).WherePropertyIsIn(allowed_states, c => c.Cities);