while($oplata = mysqli_fetch_assoc($sql_oplata)) {
$check = $_POST['oplata'] && $_POST['oplata']==$oplata['oplata_id'] ? 'checked' : null;
<p class="op"><label> <input name="oplata" class="oplata" type="radio" value="<?=$oplata['oplata_id'];?>" <?=$check;?> onclick="var el = document.getElementById('test<?=$oplata['oplata_id'];?>'); el.style.display = (el.style.display == 'block') ? 'none' : 'block';"/> <?=$oplata['oplata_name'];?><br>
<div id="test<?=$oplata['oplata_id'];?>" style="display:none"><?=$oplata['oplata_info'];?></div></label></p>
}