How long have you been doing this, and have you tried re-pitching the yeast into other batches? I am wondering if the Olive Oil is good for one generation of yeast, or if it is good for multiple generations... Very interesting.
I've done the OO thing one virtually every batch of beer using liquid yeast I've brewed in the last year or so, since I first read about the procedure...however, that's probably only 5-6 batches since I was using dry yeast quite a bit too. I drop the OO into the starter and aerate it through intermittent shaking for 8-12 hrs before it's pitched into the wort to help spread the sterols around. Also, these have been single-generation beers - so can't comment on subsequent re-pitching.
I've had nothing but vigorous, full-fermentations on all those batches with no detectable additional esters. In fact the only beer that's been 'excessively estery' was according to a judge in a recent comp I entered, but it was a batch in which I used dry yeast and the temps spiked before I knew it.
I still want to do a test, splitting a batch of wort between three fermenters and treating them as such 1.) normal wort aeration (shaking) and standard starter 2.) no wort aeration but with OO-treated starter and 3.) no aeration of wort AND no OO in the starter. The results of that regimen could be quite revealing.
There was a good thread recently on ProBrewer on OO. New Belgium evidently isn't using the process as standard, but several breweries are and getting good results.