First let's create an ExchangeManager object, where the constructor will connect to the services for us. It also has a GetOofSettings method, which will return the OofSettings object for the specified email address :

using System;
using System.Web.Configuration;
using Microsoft.Exchange.WebServices.Data;

namespace SetOutOfOffice
{
    class ExchangeManager
    {
        private ExchangeService Service;

        public ExchangeManager()
        {
            var password = WebConfigurationManager.ConnectionStrings["Password"].ConnectionString;
            Connect("exchangeadmin", password);
        }
        private void Connect(string username, string password)
        {
            var service = new ExchangeService(ExchangeVersion.Exchange2010_SP2);
            service.Credentials = new WebCredentials(username, password);
            service.AutodiscoverUrl("[email protected]" , RedirectionUrlValidationCallback);
            
            Service = service;
        }
        private static bool RedirectionUrlValidationCallback(string redirectionUrl)
        {
            return redirectionUrl.Equals("https://mail.domain.com/autodiscover/autodiscover.xml");
        }    
        public OofSettings GetOofSettings(string email)
        {
            return Service.GetUserOofSettings(email);
        }            
    }
}

We can now call this elsewhere like this:

var em = new ExchangeManager();
var oofSettings = em.GetOofSettings("[email protected]");

Using the class below, we can connect to Exchange and then set a specific user's out of office settings with UpdateUserOof:

using System;
using System.Web.Configuration;
using Microsoft.Exchange.WebServices.Data;

class ExchangeManager
{
    private ExchangeService Service;

    public ExchangeManager()
    {
        var password = WebConfigurationManager.ConnectionStrings["Password"].ConnectionString;
        Connect("exchangeadmin", password);
    }
    private void Connect(string username, string password)
    {
        var service = new ExchangeService(ExchangeVersion.Exchange2010_SP2);
        service.Credentials = new WebCredentials(username, password);
        service.AutodiscoverUrl("[email protected]" , RedirectionUrlValidationCallback);
        
        Service = service;
    }
    private static bool RedirectionUrlValidationCallback(string redirectionUrl)
    {
        return redirectionUrl.Equals("https://mail.domain.com/autodiscover/autodiscover.xml");
    }    
    /// <summary>
    /// Updates the given user's Oof settings with the given details
    /// </summary>
    public void UpdateUserOof(int oofstate, DateTime starttime, DateTime endtime, int externalaudience, string internalmsg, string externalmsg, string emailaddress)
    {
        var newSettings = new OofSettings
        {
            State = (OofState)oofstate,
            Duration = new TimeWindow(starttime, endtime),
            ExternalAudience = (OofExternalAudience)externalaudience,
            InternalReply = internalmsg,
            ExternalReply = externalmsg
        };

        Service.SetUserOofSettings(emailaddress, newSettings);
    }     
}

Update the user settings with the following:

var oofState = 1;
var startDate = new DateTime(01,08,2016);
var endDate = new DateTime(15,08,2016);
var externalAudience = 1;
var internalMessage = "I am not in the office!";
var externalMessage = "I am not in the office <strong>and neither are you!</strong>"
var theUser = "[email protected]";

var em = new ExchangeManager();
em.UpdateUserOof(oofstate, startDate, endDate, externalAudience, internalMessage, externalMessage, theUser);

Note that you can format the messages using standard html tags.