I'm going to tell a girl at work that she needs deodorant
Why would you mention to the girl that she should wear deodorant?
Because she stinks! It's not the first time I didn't like something about her.
I understand that you don't like being around people that smell, but it is inconsiderate of the girl's feelings to say out loud that she stinks. Keep a distance from this person and keep your observations quiet.
I just want to let her know that she might want to check out the deodorant section.
If there is a way to tell her discreetly, you can do that. It can be embarrassing to her if you tell her explicitly so you should try your best not to do that.