Individual TAR episodes have been repeated on a literal handful of occasions over the years; the premeire episode of the series was repeated after 9/11 after all network programming (save the second season of Big Brother) were delayed by a week due to those attacks. There was a power blackout in the Northeast one time (TAR 6, I think) that prompted a repeat the following Saturday) and individual stations might have delayed airing an episode due to local programming (such as pro or college sports games the CBS station had rights to air live in that market.)
Downton Abbey was airing on PBS, not CBS, and it was the season finale; but that plus the Daytona 500 that was rain delayed for hours, the Walking Dead, and the Olympic Closing Ceremonies was an overload, not to mention that TAR itself was delayed on the east coast by 13 minutes. Compared to all that, the Oscars may be easier competition.
As of last year March Madness is no longer an issue with TAR's scheduling (or Survivor) as both shows are now outside the broadcast schedules for that first week. The play-in games are not aired on CBS on Wednesday night; and the last of the round of 32 teams games are generally played on CBS in the afternoon. The other thing to keep in mind with the NCAA tournament is that CBS is sharing the broadcast rights with its own cable sports channel, and with Time-Warner's TBS channel, and TNT channel. Turner and CBS are sharing the later weekends of the tournament, and IIRC, there will be years where the final four may be on cable and not on CBS.