By both wRC+ (83) and wOBA (.299), the Orioles were the worst team in the American League last year when it came to hitting left-handed pitching. That's an area where Welington Castillo should help. Castillo has a career wRC+ of 126 against southpaws. The switch-hitting Matt Wieters, with a career 114 wRC+ against lefties, posted a wRC+ of just 71 in 2016. In theory, Castillo should help.
Still, despite being very right-handed, most of the O's don't produce against left-handed pitching. See the career numbers below:
Manny Machado: 111 wRC+
J.J. Hardy: 102 wRC+
Adam Jones: 95 wRC+
Chris Davis: 92 wRC+
Caleb Joseph: 63 wRC+
Jonathan Schoop: 62 wRC+
It's worth mentioning that Joey Rickard posted a 131 wRC+ against lefties in 90 plate appearances last season, but who knows what to expect from him. The same should be said of Hyun Soo Kim, who went hitless in limited duty against lefties but who almost certainly will (and should) face them more next season.
Schoop has been particularly terrible against lefties, and yet he occasionally bats second when a southpaw is on the mound.
It seems obvious that the Orioles want Mark Trumbo back. The two sides still seem to be far apart on money, and the Orioles have good reason not to give in. Trumbo had a strong offensive season last year as a power bat from the right side. But it's not even clear that he'd help in the hitting lefties category. Coming in, Trumbo had the reputation as someone who crushes lefties. And yet, he posted a wRC+ of just 71 last year. Splits can fluctuate from year to year, but it was odd to see Trumbo struggle mightily against them. If the O's bring Trumbo back, it should be as a DH only. But he's still far from a sure thing.
If Trey Mancini plays more, maybe he'll produce against lefties. Or maybe the O's will add someone else who hits lefties well, though it's frequently discussed that the Orioles want a left-handed outfielder in right field. But for the most part, it doesn't really matter if the outfielder is right- or left-handed. The Orioles need someone who's actually an outfielder, period. That guy isn't Trumbo.
The Orioles were very right-handed last year, and they could be even more so this year. Yet, despite adding Castillo, they still could be pretty awful against lefties.