There a very few WITHAM marriages in the OxFHS - Marriage Index.
The most likely for your Thomas WITHAM parents is this one.
Thomas WITHAM(of Charlton on Otmoor), to Ann Robinson(of Sparsholt, Berkshire). 11 Oct 1706 @ Oxford, St.Mary Mag.
Baptism - 15 Feb 1712 - Thomas WITHAM, son of Thomas & Ann @ Charlton On Otmoor.
Plus - Ann 1708, William 1710, Elizabeth 1715, Frances 1720. all @ Ch. o. Ot.
Steve.
From OxFHS - PR Transcripts CD - OXF-BIC01. & Oxon marriage Index.