He is interesting in that he is starting-level good at both being a pitcher and a position player. So far he hasn't shown that EITHER one of those things is at a level that will put him in the Hall of Fame.
He had a great season at the plate last year at age 27, his first season that was even better than very good. So far this season he is doing about as well at the plate for this first half as he did for the full season last year (he's behind a bit with HR pace though, and he won't hit 40 if the pace stays the same).
While he IS a power hitter, his career batting average is .264. That's not typical HOF level.
I found a list of the "15 worst Hall of Fame" members. Here are some from that list with relevant stats:
Lou Brock, .293
Ralph Kiner, .279
Phil Rizzuto...not even in for his ability at the plate, .273
Typically, a guy has a higher batting average at age 28 then they do when they call it a career, because usually there are a couple of tail off years in there.
Fred McGriff isn't in the HOF, and he has a .285 career batting average...to go with 493 HRs and 1550 RBIs that Ohtani won't reach. McGriff averaged 102 RBIs per 162 games over his entire career. Ohtani has an average of 99 right now in the peak of his career.
A far as pitching goes, he's even further away there. He's a starting pitcher with 21 career wins. Meh.
He's Dale Murphy without the MVP awards. Oh, and Murphy's career batting average was .265, higher than Ohtani's.
Maybe Ohtani will hit .280 with 35 HRs per season from now until he's 40. That would do it. HIGHLY unlikely though, and there's no reason to believe that will happen. Unless he starts doping, he has until age 35 before he starts to really tail off.