This honestly is probably the biggest question mark for both players. Neither is an elite defender in the outfield and really, doesn’t have a natural spot with the Texas Rangers that isn’t already blocked by a key player.
Castellanos started out his big league career as a third baseman for the Detroit Tigers but due to fairly underwhelming results, moved to the outfield in the 2018 season. He’s now racked up over 2500 innings in right field and while that seems to be his best fit at this point, he’s pretty below average there as well. With the Rangers planning on Joey Gallo being their right fielder of the future, it seems hard pressed to think that Texas would move him from that spot and replace him with a sub-par defender in Castellanos.
Rather, it has been reported that Texas has communicated with Castellanos about possibly playing first base, an idea to which Castellanos is open. Evan Grant first reported that idea which would see Castellanos become the primary first baseman for the club. That would likely bump Ronald Guzman either back down to Triple-A or utilize him as a late-inning defensive substitute/platoon starter when Castellanos needs a day off.
Ozuna, like Castellanos, is an outfielder but is best suited for left field defensively. Ozuna did start his career as a center fielder with the Marlins in an impressive outfield that featured himself, Christian Yelich and Giancarlo Stanton. However, since 2017, he’s been mostly a full time left fielder and it seems that is his natural fit moving forward. The Rangers, like in right field, have a long-term player currently slotted to play left field in Willie Calhoun though.
Calhoun is probably a long-term DH which means Texas could play Ozuna in left easily in 2021, but as of now, Calhoun is waiting on current DH, Shin-Soo Choo’s, contract to run out. So if the Rangers brought in Ozuna they’d have a defensive puzzle to figure out. They could move Gallo back to center field perhaps which would allow either Ozuna or Choo to play in right. But I don’t think they want to take Danny Santana out of the lineup nor do they want Ozuna or Choo playing right field everyday. Perhaps Ozuna could move to first like the reported plan for Castellanos is but that seems a bit far-fetched as does moving Joey Gallo to first base. While whatever they worked out would only have to be for the 2020 season, I question whether Texas wants to spend a year playing a bunch of people out of position.