The link between HIV and STIs goes both ways: if you have an STI you are more likely to get and pass on HIV. To understand why this is it's important to understand how HIV interacts with your body's normal response to infections.
When you get an STI, your body creates CD4 cells to fight the infection. CD4 cells are the cells that HIV needs to survive and make copies of itself. If you come into contact with HIV while your body is trying to fight another sexually transmitted infection, the virus is much more likely to be able to find and infect a CD4 cell. This helps HIV make a home for itself inside your body and start spreading the virus to other cells.
Similarly getting another STI when you are living with HIV can cause your viral load to increase. Again, this is because when your body detects the new STI, it will create more CD4 cells to fight the infection. This means that the HIV that's already in your body will have more opportunity to spread and multiply, infecting new CD4 cells that arrive in the area to fight the STI. It also triggers the CD4 cells that are already infected to start making more HIV, as they are activated to try and fight the new infection.
However, if you are adhering to antiretroviral treatment and have an undetectable viral load, studies have shown that even coming into contact with another STI won't increase your viral load, as your treatment prevents HIV from being able to make the copies needed to spread the infection.
Some STIs also create sores or cuts around your genitals. These can provide an easy entry point for HIV, so it can get into and out of your body more easily.
People living with HIV who are not on effective treatment are more likely to get other STIs and will often have more severe symptoms. This is because their body's immune system isn't able to fight the infection effectively, meaning STIs can progress more quickly, spreading and becoming more severe.
Lastly, both HIV and other STIs are passed on through unprotected sex. Therefore the same behaviours that can increase your risk of getting HIV also make you more likely to get other STIs. These include having more sexual partners, and not regularly using condoms. If you have been diagnosed with an STI recently, it's recommended that you get tested for HIV too.

Yes, although some STIs are more likely to be passed on through oral sex than others. Some of the STIs most commonly passed on through oral sex are herpes, gonorrhoea and syphilis, but other STIs can also be passed on orally.
You are most likely to pass STIs on through oral sex if the person giving oral sex has a sore throat or cuts, ulcers or sores in their mouth or on their lips. Likewise, if the person receiving oral sex has sores or cuts on their anus or genitals the risk of transmission goes up. Using a condom or dental dam for oral sex will reduce the risk of transmission.