Coming from the Open XML SDK

The Open XML SDK gives you the packages, parts and a typed class per schema element: WordprocessingDocument, SpreadsheetDocument, PresentationDocument, OpenXmlElement and its subclasses, OpenXmlReader / OpenXmlWriter for streaming. OneDocument has two ways to work at that level, and a hub above it. Read README.md first for what is true on every page.

Open, read text, edit, save

// Open XML SDK
using var doc = WordprocessingDocument.Open(path, true);
Body body = doc.MainDocumentPart!.Document.Body!;
foreach (Paragraph p in body.Descendants<Paragraph>()) { Console.WriteLine(p.InnerText); }
body.AppendChild(new Paragraph(new Run(new Text("Added"))));
doc.MainDocumentPart.Document.Save();

// OneDocument
using Document doc = Document.Open(path);
foreach (Paragraph p in doc.Paragraphs) { Console.WriteLine(p.Text); }
doc.Body.AddParagraph("Added");
doc.Save();

The hub gives the common reads and edits (paragraphs, runs, tables, styles, numbering, sections, headers and footers, fields, notes, comments, revisions, content controls). When it does not have a call, drop to the typed view of the same element:

// OneDocument: a typed view over the same markup, no conversion
var view = doc.Paragraphs.First().View;      // RecursiveAI.OneDocument.Schema.Wordprocessing.Paragraph

Differences that matter when moving code:

• No DOM load. The markup tree is lazy: a part is parsed when first touched, and a part nobody edited is written back as stored. Save() is an atomic replace.
• No Dispose of parts. The document owns everything; dispose the document.
• Unknown content is kept by construction (prefixes, mc:AlternateContent, extLst, order); you do not have to model it.
• Typed views are zero-copy wrappers, not a separate tree: editing through a view edits the markup the hub reads.
• No OpenXmlCompositeElement.Append family of validation. Write what the schema allows; the typed views create children in schema order (AddX, GetOrAddX).

Spreadsheets

// Open XML SDK
using var ss = SpreadsheetDocument.Open(path, true);
WorkbookPart wbp = ss.WorkbookPart!;
Sheet sheet = wbp.Workbook.Descendants<Sheet>().First(s => s.Name == "Sales");
var wsp = (WorksheetPart)wbp.GetPartById(sheet.Id!);
Cell cell = wsp.Worksheet.Descendants<Cell>().First(c => c.CellReference == "B2");
// ...resolve shared strings and styles yourself

// OneDocument
using Workbook wb = Workbook.Open(path);
string text = wb.Sheets["Sales"]["B2"].Text;        // shared strings, styles and dates resolved
double n = wb.Sheets["Sales"]["B2"].Number;

Streaming a large sheet

// Open XML SDK
using OpenXmlReader reader = OpenXmlReader.Create(worksheetPart);
while (reader.Read()) { if (reader.ElementType == typeof(Cell) && reader.IsStartElement) { /* ... */ } }

// OneDocument
using XlsxStreamReader book = XlsxStreamReader.Open(path);
using SheetCursor sheet = book.OpenSheet(0);
while (sheet.ReadRow()) { while (sheet.ReadCell()) { /* sheet.Kind, sheet.Number, sheet.TextUtf8 */ } }

Shared strings, styles and date detection are resolved by the cursor; it allocates nothing per cell (docs/spreadsheets-streaming.md).

Presentations

// Open XML SDK
using var ppt = PresentationDocument.Open(path, true);
SlideIdList ids = ppt.PresentationPart!.Presentation.SlideIdList!;

// OneDocument
using Presentation deck = Presentation.Open(path);
foreach (Slide slide in deck.Slides) { Console.WriteLine(slide.Title); }

Not supported or different

• No validator class. Output validation against the ECMA-376 and extension schemas is a repository test harness, not an API.
• No OpenXmlPackage.SaveAs with package-level edits through a part collection API beyond IOpcPackage (parts are added and replaced by URI).
• Typed views are generated for the schemas OneDocument pins (ECMA-376 and the Microsoft extension schemas); a namespace outside them stays reachable as markup.
• No FlatOpc helper class: Flat OPC is a save format (doc.SaveAs("x.xml")) and is read by content.