If I design a logo to sell to a client and I am using and OFL font. I have read online that I can use it for commercial purposes without having to pay licenses. I am unsure whether I have to credit the author or include the text file that comes with the font when you download it to the customer too with the logo.
You need to check the font licence to see what you can do with it.
I don't know what you mean by "download it to the customer too". Do you mean you want to give a copy of the font file to your client? If so, then that is a different thing entirely from simply using a font. Again you need to check the licence to see what is required/allowed. Usually, it's simply not allowed in most cases.
Generally a font licence allows you to use a font, for whatever purpose the licence permits, without having to credit anyone. In effect, the licence is your permit to use it. Normally, this the same whether a font is free or paid for.
Redistribution of font files is a different thing because the font file is basically a kind of software. Technically, paid-for fonts (like other software you buy) can't legally just be given to someone else. If a font is free, you could avoid the problem of redistribution by simply telling your client where they can download it. That way, you are not physically involved in the redistribution of the font file itself.
Disclaimer: I am not a lawyer. If you want legal advice, employ a lawyer.
I never supply clients with font files. Fonts I own are licensed for use only on my computer, not theirs. If a client asks you for this, you need to tell them it's not legal. If it's a free font, then you can easily just tell them where to get it without breaking any laws. If you need to explain it further to a dumb client, tell them a font license is a bit like a driving licence. You can't just let someone borrow your driving licence!