Complex sentence: John de Vere, 7th Earl of Oxford (c.12 March 1312--24 January 1360) was the nephew and heir of Robert de Vere, 6th Earl of Oxford who succeeded as Earl of Oxford in 1331, after his uncle died without issue.
Split into simpler sentences: John de Vere, 7th Earl of Oxford (c.12 March 1312--24 January 1360) was the nephew and heir of Robert de Vere, 6th Earl of Oxford. He succeeded as Earl of Oxford in 1331, after his uncle died without issue.