As noted in the official documentation, simply apply the class(es) btn btn-link:
<!-- Deemphasize a button by making it look like a link while maintaining button behavior -->
<button type="button" class="btn btn-link">Link</button>
For example, with the code you have provided:
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.1/css/bootstrap.min.css" rel="stylesheet" />
<form action="..." method="post">
<div class="row-fluid">
<!-- Navigation for the form -->
<div class="span3">
<ul class="nav nav-tabs nav-stacked">
<li>
<button class="btn btn-link" role="link" type="submit" name="op" value="Link 1">Link 1</button>
</li>
<li>
<button class="btn btn-link" role="link" type="submit" name="op" value="Link 2">Link 2</button>
</li>
<!-- ... -->
</ul>
</div>
<!-- The actual form -->
<div class="span9">
<!-- ... -->
</div>
</div>
</form>