Are the chances of getting an STD from unprotected oral sex the same as unprotected penetrative sex?

Mucus membrane is mucus membrane whether it's inside a vagina, inside a mouth, inside a rectum, or elsewhere. Anytime, anyplace body fluids come into contact with mucus membrane then infection transmission is likely. 

There is ALWAYS body fluid from a penis. Even when there isn't any male orgasm, there is still sexual fluid coming from the penis. Even when there isn't an erection, there is still sexual fluid coming from the penis. It might be in very tiny amounts, so little you don't notice it, but it's still enough to spread infection.

Use condoms for everything. 

If you have oral sex with some one that has an std, your chances are pretty good of contracting it if you don't use protection. Stds can be passed to and from the mouth and genitals, when you have oral sex.