You can use + (plus) and | (pipe or vertical bar) for advanced search.

If you wish to generate a YouTube feed with videos that mention several keywords in the title and description use + operator.

Example: cats+dogs (and cats and dogs)

Or you can simply use a space when you enter your keywords.
Example: cats dogs (and cats and dogs)

If you wish to combine searches into one single feed then you use the  | operator.
Example: cats|dogs (or cats or dogs)

Note: These operators will be guaranteed to work only in Basic settings, search by keywords.