The reason your command as written doesn't work is because you don't actually run your monitoring commands as the oracle user.
The command you give to the first su invocation, su - oracle; ps -ef | grep pmon; crsctl stat res -t, attempts to do three things in sequence as root: firstly, it calls su to spawn an interactive shell as oracle; after this shell exits, it runs the ps pipeline; finally, it runs the crsctl command. Since the monitoring commands run after the second su has finished, they run as root, which isn't what you want.
The smallest change to your command that will make it work is the following:
sudo su - root -c "su - oracle -c \"ps -ef | grep pmon; crsctl stat res -t\""
(Note that you need to escape the inner pair of quotes with backslashes so that they don't end the outer quoted string.) However, you can simplify this command significantly: the first optimization you can make is to get rid of the first su call. You don't need it, because you're already root from sudo:
sudo su - oracle -c "ps -ef | grep pmon; crsctl stat res -t"
You can still make it better, though. If sudo is configured to allow you to switch to any user (not just root), you don't need to use su at all. Instead, just specify the user you want using sudo -u:
sudo -u oracle sh -c "ps -ef | grep pmon; crsctl stat res -t"
(Note that you need to add an explicit call to sh so that you can run the entire set of commands as oracle and not just the first one. sudo's documentation claims that you can use its -i or -s options for this, but they didn't work as documented in my tests.)
If you want to keep an interactive shell as the oracle user after this command, you can simply make the last command sudo runs be an interactive shell. In this case, you probably also want to pass -i to sudo so that the interactive shell has the login environment of oracle:
sudo -i -u oracle sh -c "ps -ef | grep pmon; crsctl stat res -t; $SHELL"
