add_post_meta adds meta to one more different post

The proper way would be to check if you are on a single page, and if so, then check if your post has the oznamy set as one of its categories.

Let’s rewrite your code the following way:

if ( is_single() && in_category( 'oznamy' , get_the_ID() ) ) {
    // Get the current user
    $current_user = wp_get_current_user();
    $currentuserid = $current_user->ID;
    // Update the post's meta
    update_post_meta($post->ID , 'userid:'.$currentuserid , $post->ID); 
} 

Important notes

• This code should be used in functions.php instead of directly being used in the template files
• You might be visiting another post by accident, that causes to save metadata for that post too. You didn’t post your single.php‘s content, so I can’t tell if you have another post being loaded aside.
• You should use update_post_meta instead of add_post_meta unless you want to end up with a million of metadata saved on each visit.
• There is a second argument possible to be set in the in_category() function, which is the post’s ID. This might solve your problem, by only setting the current post’s ID. is_single() also accepts the same argument.