Otto I Herzog von Braunschweig-Lüneburg was born circa 1204 He was the son of William of Winchester Herzog von Braunschweig-Lüneburg and Helen Valdemarsdottir, Princess of Denmark. He married Matilde von Brandenburg, daughter of Albrecht II Markgraf von Brandenburg and Matilde von der Lausitz, circa 1228. He died on 9 June 1252.
Otto I Herzog von Braunschweig-Lüneburg also went by the nick-name of Otto 'The Child'.2 He gained the title of Herzog von Braunschweig-Lüneburg in 1235. Children of Otto I Herzog von Braunschweig-Lüneburg and Matilde von Brandenburg 
Johann II Herzog von Braunschweig-Lüneburg+2 d. 13 Dec 1277
Otto von Braunschweig-Lüneburg d. 4 Jul 1279
Conrad von Braunschweig-Lüneburg d. 15 Sep 1300
Adelheid von Braunschweig-Lüneburg+3 d. 12 Jun 1274
Agnes von Braunschweig-Lüneburg+ d. a 1302
Matilde von Braunschweig-Lüneburg+ b. c 1229, d. bt 1295 - 1298
Elisabeth von Braunschweig-Lüneburg+1 b. 1230, d. 27 May 1266
Helene von Braunschweig-Lüneburg+ b. c 1231, d. 6 Sep 1273
Albrecht I Herzog von Braunschweig-Lüneburg+2 b. c 1236, d. 15 Aug 1279
Otto I of Brunswick-Lüneburg (about 1204 – 9 June 1252), a member of the House of Welf, was the first duke of Brunswick-Lüneburg from 1235 until his death. He is called Otto the Child. only son of William of Winchester and his wife Helena, a daughter of King Valdemar I of Denmark. Otto married Matilda, daughter of Albert II, Margrave of Brandenburg, around 1228. 
↑ Source: The peerage Citations [S16 Jirí Louda and Michael MacLagan, Lines of Succession: Heraldry of the Royal Families of Europe, 2nd edition (London, U.K.: Little, Brown and Company, 1999), table 113. Hereinafter cited as Lines of Succession. [S38] John Morby, Dynasties of the World: a chronological and genealogical handbook (Oxford, Oxfordshire, U.K.: Oxford University Press, 1989), page 132. Hereinafter cited as Dynasties of the World.
[S37] BP2003 volume 2, page 2692. See link for full details for this source. Hereinafter cited as. [S37]
Source: S2 Title: Pedigree Resource File CD 49 Abbreviation: Pedigree Resource File CD 49 Publication: (Salt Lake City, UT: Intellectual Reserve, Inc., 2002)
Source: S3 Title: Ancestral File (TM) Abbreviation: Ancestral File (TM) Author: The Church of Jesus Christ of Latter-day SAINTS Publication: June 1998 (c), data as of 5 JAN 1998 Repository: #R1 Repository: R1 Name: Unknown
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