It depends what exactly you want, but yes that is entirely possible.
You want to add to your search query the term:
That will restrict your results to only include results who's url contains the path specified, or in other words, who are part of the specified site (or site collection) or it's sub-sites. To search multiple site collections at once, use:
(path:http://domain/site1 OR path:http://domain/site2 OR path:http://domain/site3)
If you want to save patterns like this, you can save it as a Result Source, either in central admin (does o365 give you a central admin? I'm too used to on premises) or in site settings for a specific site collection. If you need it to be accessible from a number of places, but can't set it application wide, set up a new search centre site collection, create the result source there, and set each of the other site collections you want to use that result source to send queries to the search centre (under Search Settings in Site Settings)