What it sounds like
Frequency grouping, Once-off, radio button, not checked, 1 of 2
This is good because the user knows the name of the group of controls and how many there are, but sighted users may still be confused.
What it looks like
<fieldset>
<legend class="visuallyhidden">Frequency</legend>
<input type="radio" id="once-off" name="repetition" value="Once-off" />
<label for="once-off">Once-off</label>
<input type="radio" id="repeating" name="repetition" value="repeating" />
<label for="repeating">Repeating</label>
</fieldset>