Find Oracle Home for PMON Process

$ ps -ef | grep pmon

ora1024   262366        1   0   Mar 23      -  0:12 ora_pmon_mysid

ORACLE_SID is mysid

$ ls -l /proc/262366/cwd

lr-x------   2 ora1024  dba  0 Mar 23 19:31 cwd -> /data/opt/app/product/10.2.0.4/db_1/dbs/

ORACLE_HOME is /data/opt/app/product/10.2.0.4/db_1