Can a company do SEO by itself?

Definitely.  SEO was made for companies that work hard and are willing to grind to increase their visibility.  Don’t get me wrong.  It will be a LONG process.  It takes a ton of work to get where you want.  Many companies will promise you a first page ranking on Google but this requires a lot of work.  My recommendation…if you’ve just started a company and have limited resources or even if you’ve been around for awhile and just want to improve your station, it’s best to consider an SEO professional.