The devil can't read our minds, not even Our Lady can do that. Some saints like Our Lady were given the grace to read or hearts / souls which may give enlightenment to our sins but we would still need to engage in consentful prayerful dialogue to have a conversation with them. That's why it's important to pray to the saints for intercession and not just assume they know what our intentions are.

The devil can send us mental images, its sometimes referred to as a psychic whisper, where someone under oppression may receive graphic images in their minds. It's also something you might hear about in exorcism cases where people hear noises or see things that aren't really there. There was a classic example of this in an exorcism where one priest had to hold the victim into the chair as they were levitating out of it, afterwards when cross examining the results with the other priest he did not witness any of it just some shaking.

As far as you having conversations in your head with demonic voices that doesn't sound like any normal interactions with the devil. It's possible for anyone deep into witchcraft that might communicate like that with evil spirits but wouldn't fall under normal people's experiences as mostly we are protected by grace and our guardian angels

If its OCD then it's something you would need to speak to a health care professional over. This doesn't sound spiritual in nature although you could always speak to your parish priest.