Complex sentence: His family then moved to Edinburgh, where he completed his secondary education at Holy Cross Academy, before studying at the University of Edinburgh where he gained a Bachelor of Science degree in chemistry in 1959 (and later a Diploma in Education in 1966).
Split into simpler sentences: His family then moved to Edinburgh, where he completed his secondary education at Holy Cross Academy. O'Brien then studied at the University of Edinburgh where he gained a Bachelor of Science degree in chemistry in 1959 (and later a Diploma in Education in 1966).