To SSH into an instance as the 'ubuntu' user and execute commands requiring root access using sudo, follow these steps:

1. Open your terminal.

2. Use the ssh command to connect to your instance, specifying the key pair file for authentication, and replace <your-instance-IP> with your instance's public IP address:

   ssh -i /path/to/your/private-key-file.pem ubuntu@<your-instance-IP>
   

   Make sure to replace /path/to/your/private-key-file.pem with the actual path to your private key file.

3. You will be prompted to confirm the connection by entering 'yes' if it's your first time connecting to the instance.

4. Once connected, you will be logged in as the 'ubuntu' user.

5. To run commands requiring root access, use sudo followed by the command. For example, to update the package list:

   sudo apt update
   

   You will be prompted to enter the 'ubuntu' user's password for elevated privileges.

Now you can execute OS commands on your Ubuntu instance via SSH as the 'ubuntu' user with sudo access.