I think that depends on the designer. Some of them understand OLM and some don't.
Ask for credentials in terms of both - a good designer will design for Search Engine Optimization, but there is a lot more to Online Marketing than that - such as link building - which a designer may not understand. Online Marketing is also an ongoing endeavour - the search engines like to see activity around your site.
So best to ask them for examples of sites they have both built AND driven qualified traffic/ leads to.