These tools can help you determine whether the item you want to use is in the public domain or not. If it's in the public domain, you don't need permission to use it. Remember to be mindful of other issues, such as local and cultural norms. You may choose to obtain permission even if not legally required.
For items under copyright, the following tools will help you determine if your use would fall under Fair Use. If your use falls under Fair Use, you don't need to obtain permission, but Fair Use is a judgement call, so you may choose to obtain permission anyway. Also, as above, other factors may impact your decision whether or not to ask for permission.
Copyright may be a factor with OWL content, before, during, and after an event. While perhaps not directly applicable to your intended uses, these codes of best practices may help you determine whether or not material can be used, whether access should be open or limited, whether or not it's ok to record the event and post the recording, and what rights you and your presenters may have.
There are many licenses besides those listed here. Some may be standardized, and some not. Be sure to check the terms of any material that may be licensed to you or your institution before using it. When licensing your own material, consider your terms carefully.