var doc = PdfDocument.Load(@"d:\1\SimplifiedExamplePdf.pdf");
var page = doc.Pages[0];
foreach(var obj in page.PageObjects)
{
	foreach(var mc in obj.MarkedContent)
	{
		if(mc.Tag=="OC") //This is an optional content
		{
			if (mc.Parameters != null)
				Console.WriteLine((mc.Parameters["Name"] as PdfTypeString).UnicodeString);
		}
	}
}

...
ABA-WD-WALL HATCH
ABA-WD-WALL HATCH
ABA-WD-WALL HATCH
ABA-WD-WALL HATCH
ABA-WD-WALL PLASTER
ABA-WD-WALL PLASTER
ABA-WD-WALL PLASTER
ABA-WD-WALL PLASTER
ABA-WD-WALL PLASTER
ABA-WD-WALL PLASTER
ABA-WD-DOOR EXERNAL
ABA-WD-DOOR EXERNAL
ABA-WD-DOOR EXERNAL
ABA-WD-DOOR EXERNAL
ABA-WD-DOOR EXERNAL
ABA-WD-DOOR EXERNAL
ABA-WD-DOOR EXERNAL
ABA-WD-WINDOWS
ABA-WD-WALL EXTERNAL
ABA-WD-WALL EXTERNAL
ABA-WD-WINDOWS
ABA-WD-WINDOWS
ABA-WD-WINDOWS
ABA-WD-WALL PLASTER
ABA-WD-WINDOWS
ABA-WD-WINDOWS
ABA-WD-WALL HATCH
...

static void Main(string[] args)
{
	PdfCommon.Initialize();
	var doc = PdfDocument.Load(@"d:\1\SimplifiedExamplePdf.pdf");
	var ocgObj = GetOcgFromObject(doc);
	var ocg = GetOcgFromOCProperties(doc, ocgObj.Handle);
	bool itIsTrue = ocgObj.Handle == ocg.Handle;
}

public static PdfTypeDictionary  GetOcgFromOCProperties(PdfDocument doc, IntPtr  handle)
{
	var root = doc.Root;
	var oc_prop = GetDictionary(root, "OCProperties", null, false);
	var ocgs = oc_prop["OCGs"] as PdfTypeArray;
	foreach (var item in ocgs)
	{
		var ocg = (item as PdfTypeIndirect).Direct as PdfTypeDictionary;
		if (ocg.Handle == handle)
		{
			return ocg;
		}
	}
	return null;
}

public static PdfTypeDictionary GetOcgFromObject(PdfDocument doc)
{
	var page = doc.Pages[0];
	foreach (var obj in page.PageObjects)
	{
		foreach (var mc in obj.MarkedContent)
		{
			if (mc.Tag == "OC") //This is an optional content
			{
				return mc.Parameters;
			}
		}
	}
	return null;
}

public static PdfTypeDictionary GetDictionary(PdfTypeDictionary parent, string entry, PdfIndirectList list = null, bool is_create = true)
{
	PdfTypeDictionary ret = null;
	if (!parent.ContainsKey(entry))
	{
		if (!is_create)
			throw new Exception(string.Format("The document does not have {0} entry", entry));

		ret = PdfTypeDictionary.Create();
		if (list != null)
		{
			list.Add(ret);
			parent.SetIndirectAt(entry, list, ret);
		}
		else
			parent.SetAt(entry, ret);
	}
	else if (parent[entry] is PdfTypeIndirect)
		ret = (parent[entry] as PdfTypeIndirect).Direct as PdfTypeDictionary;
	else if (parent[entry] is PdfTypeDictionary)
		ret = parent[entry] as PdfTypeDictionary;
	else
		throw new Exception(string.Format("'{0}' entry has unknown type", entry));

	return ret;
}