I'm planning a party and want to make multiple mug cakes, but since I need to make about 100 and don't have a microwave that big I was wondering if I could make them in the oven and if there were oven safe mugs. I've been looking but they all seem to say dishwasher and microwave safe but nothing about the oven. Would it be easier to make them in the microwave one at a time? Just seems like a hassle.
A microwave has a very different mechanism of heating than an oven, and recipes for the two are not interchangeable. The results of making microwave-optimized cakes in the oven will be unpredictable. And this will be a much larger problem than having the right mugs.
A better option would be to find a recipe for muffins or cupcakes that is intended to go in the oven, and bake it in the oven. Do test it in a small batch before giving it to 100+ guests, because not all recipes floating around are good - but still, it is more likely to produce good results than baking your microwave cakes in an oven.
Not all mugs are oven safe.
I do have a couple that are from corningware that say specifically that they're used in the oven. Mine are 20oz, but I've found 11oz ones on Amazon that are about $4.12 each if buying in lots of 6. (so 17 6-packs would be $415.82+tax to get 102 mugs). Other places might have better prices (I know that's true for the 20oz ones).
It's also possible that restaurant supply stores might have something cheaper.
If you already have your 100 mugs, and they're all the same manufacturer & model, you can try testing one of them ... but put a sheet pan on the shelf below just in case it explodes to try to contain the mess.
If they do explode in the oven, you could run a trial in a water bath, but that's likely to affect cooking times dramatically and be more dense. (pour in near-boiling water after the cups are in place for best results ... but then you also have to be careful about removing them (I'd recommend canning tongs). For working in bulk, if you're really careful about sliding the shelf out so you don't slosh the water into the water or on yourself, you might be able to pull the cups, then add back in a new batch, replace any evaporated water, then bake.
As for microwaving, you could try doing it that way. If you have a carousel and can fit a few on there, I'd try arranging them in a circle, microwaving about half way through, rotating them 180 degrees, and then finish cooking. (but test this out before hand, so you can figure out proper time as it's likely to be longer than just cooking one (and to see if it even works)
A big part of microwave mug cakes is warm, freshly-baked cakes, and you'd expect to be able to hold the handle. So if you cooked them in the oven you would have to let them cool quite a lot before serving.
And the mug cake recipes I've eaten aren't that good when they've cooled down (some compromises must be made to make them work in the microwave I guess, or maybe it's an effect of the microwaving process). So if you go for it I suggest a normal cupcake recipe.
While there are good reasons to be wary of some mugs in the oven, if you get plain (not painted) glazed mugs with the glaze undamaged, you should be able to just put them straight in.
If you want to try the mugs you already have (or can buy), you should test it beforehand. You might put them into a cold oven, and let them warm up there and cool in the oven afterwards - it might not be best for the cake, but it will reduce the thermal shock to the mugs.
Another possibility you may not have thought of - you can make cupcakes, and settle them into the mugs afterwards (might take a bit of trimming to make fit). Or make a full cake and cut pieces out to size - most of the joins will be covered over by the mug itself, if you do layers or fill one or two with all edges or something, as long as the top looks good (which can be neat and careful cutting, or else frosting or some other topping, your choice).
Your mugs should almost certainly stand up to gentle warming in the oven, as long as it doesn't get too hot (and you will want the mugs cool enough to handle, anyway). Or you can serve it cool, if the "in mugs" part is the important bit.
I admit I got both answers from the oven-safe drinking glasses question.