I think Olson is their second best pitcher, so I would guess he's safe unless he looks terrible in Spring training. Anyway, at the end of last year Hinch or Harris said something about not having a traditional five man staff, but rather having a couple of swing men. So, I wouldn't be at all surprised for them to go with six starters. It would still be pitchers pitching with four days rest for the most part, but there might be a lot of split starts.
The most surprising thing might be all six being healthy on opening day! So, I am happy to see some depth.