Yes, I use them all the time to flavor my buttercream. You need to be very careful about the amount you use. And some of the flavors will color your icing, so be wary of that. The raspberry, for example, will give you a pink color in your icing. I would much rather use some of the oil than use some kind of juice. I don't keep pomagranite juice around, for example, but sometimes I want that flavor in icing.
as the previous poster said be very careful about how much you add at a time. I added a dram to a double batch of buttercream one time and it was so sickly sweet and candy tasting it got thrown out, i probably could have used 1/4-1/2 of the dram and had much better results.
As for the coloring, i have found (and quite by accident) that if you leave the bottles in the sunlight it will turn them clear. I have not had any issues with the flavor being diluted or changing when doing this. i don't know if it shoudl change the flavor or if i just got lucky, but it was a happy mistake i found out.
leily~
Did it change the quality of the oils...as far as in flavor? Just curious...I have a few bottles of mango that are orangish in color and one that is clear. I wondered if they forgot to add the coloring but maybe it was just left in the sunlight...hmmm...
I have used the Lorann oils in buttercream for cupcakes. I made a double batch of my buttercream so I used an entire dram and it came out delicious. I used the grape flavoring on yellow cake cupcakes for my husband's co workers and from what I hear, everyone raved over them. I plan on using either my cherry or butterscotch flavorings next. 
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