Using a select form field for the field that selects the groups where a node is posted
I am replying basing on what you wrote in the comment to your question.

Organic Groups shows checkboxes or a select field basing on the number of groups that are listed in the "Audience" form field. If the number of groups is less than 20, the module will use checkboxes; otherwise, it will use a select box.

There is no way to change which form field is used, except using a module that alters the form fields added by Organic Groups. The following code, contained in og.module, is the code that adds the audience form field to the node edit form.

// show multi-select. if less than 20 choices, use checkboxes.
$type = $cnt &gt;= 20 || $is_optgroup ? 'select' : 'checkboxes';
$max_groups = variable_get('og_max_groups_'.$node-&gt;type,'');
$description_max_groups = ($max_groups &amp;&amp; !user_access('administer nodes')) ? format_plural($max_groups," Limited to !max_groups choice."," Limited to !max_groups choices.", array('!max_groups' =&gt; $max_groups)):'';
$form['og_nodeapi']['visible']['og_groups'] = array(
  '#type' =&gt; $type,
  '#title' =&gt; t('Audience'),
  '#attributes' =&gt; array('class' =&gt; 'og-audience'),
  '#options' =&gt; $type == 'checkboxes' ? array_map('filter_xss', $options) : $options,
  '#required' =&gt; $required,
  '#description' =&gt;  format_plural(count($options), 'Show this post in this group.', 'Show this post in these groups.') . $description_max_groups,
  '#default_value' =&gt; $groups ? $groups : array(),
  '#required' =&gt; $required,
  '#multiple' =&gt; TRUE);


It is enough to have a module that executes the following line, and the form field will always use a select box to show the list of groups to which associate a post.

$form['og_nodeapi']['visible']['og_groups']['#type'] = 'select';


The code needs to be executed inside the implementation of hook_form_alter(), or hook_form_FORM_ID_alter().