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Well, image scale yes, or your wasting the res at 0.5asp with bad guiding, but also FL counts.
Thats the funny thing actually, OAG is easier with a refactor, but its not so important (wide field,but if you can easily, as Alex does, why not!), a refractor guide scope on a refractor imager bolted down well wont have much flexure cause they both flex the same way ie they are both heavier at the same end, and the average mount can handle a 2 refractor load better.
Its at long FLs with big heavy SCTs, you NEED OAG cause flexure becomes very obvious, but its much harder, as guide stars are harder to find, and dimmer, and a typical SCT is heavier at the opposite end than a refractor guide scope, making flexure worse.
I found at 2m FL (or less) F6.7, an external guide scope was ample and very convienient if the guide scope/cam were all bolted down hard, not adjustable, to 20 min exposures, just.
Now at 2270mm, and 20min exposures (.62asp image scale) I have to use OAG, and its a serious pain in the neck.......... until lately I found the secret of success, took a long while, but well worth the effort IMO.
I spent hrs at times, even with an auto rotator, tring to find a guide star randomly and preserve a set composition, very frustrating. The FOV indicator in the sky was wrong, the SBIG internal guide box was nothing like reality, so I tried like hell to make the FOV indicator see the same as the OAG. In the end, I took long guide exposures, plate solved them, and compared them to the plate solved main image, the OAG pic was WAY off, like 3 times as far as the internal guide chip, and 180deg rotated !. I modified the FOV indicator, with painfull trial and error, untill it was dead right.
Now, finding a guide star is so stupidly quick, you adjust the FOV indicator so it shows the view you want (in the sky), with a guide star where you want it, nudge the scope image to the same view, and voila, instant guide star, ready to go. I was so often sooo close to a guide star randomly looking, but always just out of view without knowing. There are plenty of obvious guide stars for the picking in the sky, but near impossible to find just randomly poking around.
With the rotator linked to the sky, its a joy just to rotate the sky FOV indicator to taste, the rotator follows, and bingo, a guide star pops out just where the sky FOV indicator says it should be. And, you can select the one you want in advance based on magnitude shown in the sky.
This all is just as relivant, in fact more, if you dont have a rotator, just set the sky FOV angle to your OAG angle manually in the sky, and nudge the scope to position.
Never again will I randomly poke around for a guide star, its well worth the effort to get the sky FOV indicator just right..........NOW its nearly as convienient as a guide scope ;-).
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