You can use <s> element:

Parad<s>o</s>x Jewelz

Or inline-style, or create a css class for letter o only, see the example below

.strikethrough {
  text-decoration: line-through;
}

<p>Parad<s>o</s>x Jewelz</p>

<p>Parad<span style="text-decoration: line-through;">o</span>x Jewelz</p>

<p>Parad<span class="strikethrough">o</span>x Jewelz</p>

Use the CSS attribute text-decoration and value line-through.

.strikethrough{
  text-decoration:line-through;
}

<p>Parad<span class="strikethrough">o</span>x Jewelz</p>

Or you can simply use the <s> element

Parad<s>o</s>x Jewelz

Or <strikethrough>, although that’s obsolete (why type more characters when you don’t need to?)

I want to do a strike-through text effect for one letter.

How could I achieve that with CSS or HTML?