Hi sorry if there is already a topic about this but I was wondering how OCs should be tagged and if it's possible to make new tags for specific OCs if there is more then image of them and if there isn't a tag yet for them.

If a character is in the image and the character has a name, by all means, feel free to tag it!

Generally, the format is:
If multiple characters exist with the same name, tag it like character_name_(owner_name)
If one character exists with a unique name, it is just simply tagged character_name.

If the tag isn't already a character tag, you will need to tell the site that "This is a character tag" by "casting" that tag into a character tag, so say for example, if I were to post my character for the first time, and my character shares a name with multiple characters, I would tag it like so:
character:félix_(softhyena.com)
character: meaning "This is a character tag, please cast it into the character tag type"
félix being my character's name
_(softhyena.com) since félix is also my username, it would only make sense to tag it with something uniquely identifiable, so I went with softhyena.com since that is my webpage.
If it is already cast into a character tag, you can simply just tag posts with my character as félix_(softhyena.com) and it will still be a character tag.

If you have any more questions about this, feel free to ask, and thank you for taking time to ask how to tag correctly! :D

To clarify a bit further what Chaser is saying, if somebody were to try to tag character:félix, it would result in all pre-existing tags, which are currently artist type, being converted to character types, not "just" generals. This is different from some booru sites such as derpibooru, where the tag will hang onto the artist: prefix so that you can have artist:félix and character:félix at the same time.