Another easy possibility would be to wrap the list item content into a <p>, style the <li> as bold and the <p> as regular. This would be also preferable from an IA point of view, especially when <li>s can contain sub-lists (to avoid mixing text nodes with block level elements).
Full example for your convenience:
<html>
<head>
<title>Ordered list items with bold numbers</title>
<style>
ol li {
font-weight:bold;
}
li > p {
font-weight:normal;
}
</style>
</head>
<body>
<ol>
<li>
<p>List Item 1</p>
</li>
<li>
<p>Liste Item 2</p>
<ol>
<li>
<p>Sub List Item 1</p>
</li>
<li>
<p>Sub List Item 2</p>
</li>
</ol>
</li>
<li>
<p>List Item 3.</p>
</li>
</ol>
</body>
</html>
If you prefer a more generic approach (that would also cover other scenarios like <li>s with descendants other than <p>, you might want to use li > * instead of li > p:
<html>
<head>
<title>Ordered list items with bold numbers</title>
<style>
ol li {
font-weight:bold;
}
li > * {
font-weight:normal;
}
</style>
</head>
<body>
<ol>
<li>
<p>List Item 1</p>
</li>
<li>
<p>Liste Item 2</p>
<ol>
<li>
<p>Sub List Item 1</p>
</li>
<li>
<p>Sub List Item 2</p>
</li>
</ol>
</li>
<li>
<p>List Item 3.</p>
</li>
<li>
<code>List Item 4.</code>
</li>
</ol>
</body>
</html>
(Check the list item 4 here which is ol/li/code and not ol/li/p/code here.)
Just make sure to use the selector li > * and not li *, if you only want to style block level descendants as regular, but not also inlines like "foo <strong>bold word</strong> foo."
The proposed solution also works when the text is longer (i.e. the lines need to wrap): Updated Fiddle
When you're using a grid system, you might need to do one of the following (at least this is true for Foundation 6 - couldn't reproduce it in the Fiddle):
Add box-sizing:content-box; to the list or its container
OR change text-indent:-2em; to -1.5em
P.S.: I wanted to add this as an edit to the original answer, but it was rejected.
<ol class="font-weight-bold">
<li><span class="font-weight-light">Curabitur aliquet quam id dui posuere blandit.</span></li>
<li><span class="font-weight-light">Curabitur aliquet quam id dui posuere blandit.</span></li>
</ol>
Answer https://stackoverflow.com/a/21369918/2526049 from dcodesmith has a side effect that turns all types of lists numeric.
<ol type="a"> will show 1. 2. 3. 4. rather than a. b. c. d.
The above code adds support for lowercase letters, lowercase roman numerals. At the time of writing browsers do not differentiate between upper and lower case selectors for type so you can only pick uppercase or lowercase for your alternate ol types I guess.