Why is output for grep -o the same with or without LC_ALL=C? There is a difference for grep with no flags for me as expected but there's no difference for grep -o. Does grep -o always use LC_ALL=C or something else?

[aa@bb grep-test]$ cat input.txt
aa bb
CC cc
dd ee

[aa@bb grep-test]$ LC_ALL=C grep -o [A-Z] input.txt
C
C
[aa@bb grep-test]$ grep -o [A-Z] input.txt
C
C
[aa@bb grep-test]$ LC_ALL=C grep [A-Z] input.txt
CC cc
[aa@bb grep-test]$ grep [A-Z] input.txt
aa bb
CC cc
dd ee
[aa@bb grep-test]$ locale
LANG=en_IE
LC_CTYPE="en_IE"
LC_NUMERIC="en_IE"
LC_TIME="en_IE"
LC_COLLATE="en_IE"
LC_MONETARY="en_IE"
LC_MESSAGES="en_IE"
LC_PAPER="en_IE"
LC_NAME="en_IE"
LC_ADDRESS="en_IE"
LC_TELEPHONE="en_IE"
LC_MEASUREMENT="en_IE"
LC_IDENTIFICATION="en_IE"
LC_ALL=
[aa@bb tmp]$ grep -V
GNU grep 2.6.3
...
[aa@bb tmp]$ bash --version
GNU bash, version 4.1.2(1)-release (x86_64-redhat-linux-gnu)