$ORACLE_BASE/diag/rdbms/{DBNAME}/{sid}/trace/alert_{sid}.log

ASM alert.log can be found in:

$ORACLE_BASE/diag/asm/+asm/+ASM{instance number}/trace/ alert_+ASM{instance number}.log

Clusterware CRS alert.log can be found in:

$GRID_HOME/log/{node name}/alert{node name}.log