A TM protects the holder's products & reputation from being harmed by inferior or confusing products. The problem with TM & domain names is that most TMs are specific to a particular product or business type. Thus, real words such as names can be TMed in multiple businesses. Ex.: Black Mountain Ranch breeds horses, Black Mountain Winery produces a fine Shiraz, and Black Mountain Books publishes maps. All 3 get to TM "Black Mountain" & can even have nearly identical logos, because the "average consumer" is not going to think that any of the 3 businesses are related. No one's TM gets weakened by allowing multiple Black Mountain businesses.
McDOnline.com sounds very much like an official Mcd's site. Therefore, if you put anything on the site that talks about McD's, you'll likely be found to be infringing on McD's TM. If, however, your site refers to Clan McDonald, greatest Scotsmen ever to go regimental, you're fine.
In fact, you even have a good argument that you need to add the 'online' bit because most people looking for the restaurant chain will type Mcd's.com and you don't want them finding your site by mistake. Slick huh?
Furthermore, if you own McDOnline.com and aren't using it for some legitimate, non-infringing purpose, ICANN will assume that you were cyber-squatting, intending to hold up McD's for all you could. ICANN will take away your site and let McD's have it. BUT (this is the really good part), if you've established a legitimate non-infringing site & McD's wants to take it over and turn it into an official McD's site, they have to buy the site from you.
This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.