Complex sentence: O'Brien planned to swim the race at an even pace and record even splits for the first and second half of the race, but he was aware of not chasing Jastremski, who was known for an aggressive opening style, which resulted in a faster first half.
Split into simpler sentences: O'Brien planned to swim the race at an even pace and record even splits for the first and second half of the race. He was aware of not chasing Jastremski, who was known for an aggressive opening style, which resulted in a faster first half.