it should already be implented, so all you need to do is 2 things
first in say register.php put this
<?php osc_run_hook('user_register_form'); ?>
<script type="text/javascript">
var RecaptchaOptions = {
theme : 'custom',
custom_theme_widget: 'responsive_recaptcha'
};
</script>
<div id="responsive_recaptcha" style="display:none">
<div id="recaptcha_image"></div>
<div class="recaptcha_only_if_incorrect_sol" style="color:red">Incorrect please try again</div>
<label class="solution">
<span class="recaptcha_only_if_image">Type the text:</span>
<span class="recaptcha_only_if_audio">Enter the numbers you hear:</span>
<input type="text" id="recaptcha_response_field" name="recaptcha_response_field" />
</label>
<div class="options">
<a href="javascript:Recaptcha.reload()" id="icon-reload">Get another CAPTCHA</a>
<a class="recaptcha_only_if_image" href="javascript:Recaptcha.switch_type('audio')" id="icon-audio">Get an audio CAPTCHA</a>
<a class="recaptcha_only_if_audio" href="javascript:Recaptcha.switch_type('image')" id="icon-image">Get an image CAPTCHA</a>
<a href="javascript:Recaptcha.showhelp()" id="icon-help">Help</a>
</div>
</div>
<?php osc_show_recaptcha(); ?>
you might already have run hook register so you may find this easier
then upload the responsive_recaptcha.css somewhere in your files and at the bottom of say register.php point a src to it like
<link rel="stylesheet" href="
https://www.YOURSITE/css/responsive_recaptcha.css">
AND AWAY YOU GO