It's pretty safe to assume he has Jason Hammel and Wei-Yin Chen written with the pen while using the pencil for Miguel Gonzalez and Chris Tillman. Four spots seem pretty set although Gonzalez and Tillman likely have a less firm grip on a spot than the the first two at this point.
So that leaves as many as 10 others that have at least an outside shot at a rotation spot. Ten were mentioned in the Stark article in addition to the first four.
Here is how I rank them, as of today, to gain the fifth spot from most likely to least likely:
Zach Britton- Still a big fan of the power sinker. Looks healthy and if O's want two lefties in the rotation, this could be the guy.
Jair Jurrjens- If the right knee holds up, should be a strong candidate.
Steve Johnson- In small sample, was just too good last year to overlook.
Jake Arrieta- Still a Jake fan, but he has to prove he can get it back after a three-year ERA increase.
Brian Matusz- Still feel he will end up in the bullpen, but he's another lefty option and if he can throw like he once did as a starter, he could claim the job.
Dylan Bundy- He's good and he's close to ready. When will it be his time?
Kevin Gausman- Fans are going to be very impressed with his fastball-changeup combo. The change could be his best pitch actually.
Tommy Hunter- Very intrigued by big velocity increase when he pitched out of the bullpen last September.
T.J. McFarland- A longshot right now but maybe odds are better that he can grab a bullpen role.
Tsuyoshi Wada- Seems ahead of schedule from the surgery, but the opening day rotation sounds too ambitious at this point.
Keep in mind a few things as this competition gets going. This will be like a long horse race. The horse that breaks fast and leads at the quarter pole sometimes finishes last and often there are fast finishers that win the race.
Spring training stats can be misleading. Because the hurlers pitch so few innings in camp, one bad outing can really impact an ERA, for instance. Plus, some of this group may pitch in "B" games or minor league games and we won't have any stats to factor in from those outings.
Some that lose out for rotation spots could start out in the 'pen either in Baltimore or Norfolk and still might be key factors on the pitching staff.
Of the 10 pitchers listed above, Hunter and Wada are out of options. I would guess Wada could begin on the disabled list and pitch in the minors on a rehab assignment. McFarland, as a Rule 5 pick, must be offered back to Cleveland if he doesn't make the opening day roster.
I can't imagine a scenario where one of the above would pitch well, but miss out on the rotation and then be exposed to waivers. It's much more likely they would just have to start the year in the minors.
By the way, the Orioles are scheduled for seven games in seven days to start the year so the first day a fifth starter is needed is for the season's fifth game on April 6 at home against Minnesota. I would guess the O's will use that roster spot on someone else for the first four games and then recall someone to make the April 6 start.