The young open cluster NGC 6231 hosts a rich population of O-type binary stars. We study several of these eccentric short-period massive eclipsing binaries and assess their fundamental parameters. The properties of these systems make them interesting targets to study tidally induced apsidal motion. The analysis of apsidal motion offers a powerful means to obtain information about the internal structure of the stars. Indeed, since the rate of apsidal motion in a binary system is proportional to the internal structure constants of the stars composing it, its value gives direct insight into the internal structure and evolutionary state of these stars. Stellar evolution models are constructed based on the observationally-determined fundamental parameters and a theoretical rate of apsidal motion is inferred. The results are striking: Adopting standard stellar evolution models yields a theoretical rate of apsidal motion much larger than the observational value. This discrepancy results from the standard models predicting too low an efficiency of internal mixing and thus too homogenous stars in terms of density. By enforcing the theoretical rates of apsidal motion to match the observational values, enhanced mixing is required, through a large overshooting parameter and/or additional turbulent/rotational mixing. Our analysis leads to the conclusion that the chemically mixed cores in those massive stars must be more extended than anticipated from standard models.