For this bug (#33080) it seems like -o would have to be taken into consideration before -c.
For bug #23635 Anonymous would like grep -C2;grep -C2 -v to output every line of the file. Currently it may include duplicates. This is similar and -C would have to be taken into consideration before -v.
If I'm not mistaken -C (as mentioned in #23635) and -o are gnu extensions but i haven't looked at the source and have no intention to. Only based off the results i assume gnu extensions are done last.
Here is another proposal for intuitive operation. Currently grep -ov will always return nothing which is not very useful. But what would be useful would be to return the nonmatching portion of the matching lines. This would give functionality, similar in many ways to #29827 but that is overcomplicated in my opinion. I am not prepared to write a patch at this time.
I hope you will understand and respond.

--only-matching (-o) causes grep to print all the matching parts separately, no matter how many times such parts occur on a single line. For example:

$ cat > test.txt
true true false
false true
false
true false true
$ grep --only-matching b test.txt
true
true
true
true
true

If you want to count how many matches you have, you naturally think you can use --count (-c). But you can't:

$ grep --only-matching --count true test.txt
3

You still get the number of "lines" that match the pattern, not how many "matches". The --only-matching option is effectively ignored, in a silent way.

This is counter-intuitive, when you consider that 'grep --count ...' is generally equivalent to 'grep ... | wc -l', which doesn't hold if --only-matching is used.

$ grep true test.txt | wc -l
3
$ grep --count true test.txt
3
$ grep --only-matching true test.txt | wc -l
5
$ grep --only-matching --count true test.txt
3

I suggest that grep --count should print the number of lines that would be printed without --count, whether or not --only-matching is used; that is, the number of lines that match the pattern without --only-matching, and the number of matches with --only-matching.