That one is not so good. You will DEFINATELY have to tell them because you will have to report all employers (including military). OCS is designed to mostly test personality, motivation, ethics, leadership, and stress handling all of which are relevant to the airlines (especially captains). Any sort of "suitability drop" would worry me, as well as an academic failure, because OCS academics are not that hard compared to 121 training (which is also conducted under stress).

If you failed the underwater swim or pushups, that will not be a problem. Anything else, you need to come up with a good explanation and be prepared to talk about what you learned from the experience.