you could use the -c option http://linux.die.net/man/1/su
su - l oracle -c "export ORACLE_SID=MYDB && sqlplus user/password"
Personally, I'd expect the oracle acct to already have ORACLE_SID setup in the login. Also, if you're going to automate this, I'd put all the SQL into one file, so something like
su -l oracle -c "sqlplus user/password @mysqlfile.sql"
The only issue is security; do you really want user/passwd in a shell file?