Ohtani is the second player to make All Star teams as both a pitcher and a position player*. The first was Junzo Sekine, who played for Kintetsu in the 1950's.
I will caveat this by saying that while Sekine and Ohtani were the only players who were SELECTED to the team as both a pitcher and a position player, they aren't the only ones to play both positions in All Star games. Lions pitcher Kimiyasu Kudoh played outfield in one of the 1990 games after an injury left the PL short position players in an extra inning game and Ichiro famously pitched to one batter in one of the 1996 games.
Sekine attended Hosei University just after the war. He pitched a 12 inning complete game loss to the San Francisco Seals in their tour of Japan in 1949.
|2011 BBM Legend Of Tokyo Big 6 #048|
He joined the then Kintetsu Pearls in their inaugural at the request of their manager, Shozo Fujita. Fujita had been Sekine's coach at Hosei. He went 65-94 over the next eight seasons for a mostly lousy Kintetsu team (they finished 7th in the then seven team Pacific League from 1950-1953 and never finished higher than fourth during that time. That fourth place finish in 1954 was the only time the team was over .500 until 1963). He made the All Star team as a pitcher in 1953.
|~1956 Bromide (might be JBR 42)|
|1959 Marusan JCM 12a|
|1962 Doyusha JCM 55|
|2006 BBM Nostalgic Baseball #086|
Following his playing days, Sekine was a coach for the Carp (1970) and Giants (1975-76). He also managed for six seasons in the 1980's - three with the Whales (1982-1984) and Swallows (1987-89). He was elected to the Hall Of Fame in 2003.
|2008 BBM Yokohama 30th Anniversary #03|
|2009 Epoch All Japan Baseball Foundation #33|
Gary Garland's Japan Baseball Daily website may be gone, but you can still see it at the Wayback Machine Internet Archive and so I was able to use it to research this post. I also used Sekine's page on the Japanese version of Wikipedia.