If you take baby powder or baking soda (dime size amount) into your palm and put it on your finger tips, massage it on your scalp and it will soak up the oil. But this is mainly to get rid of the oil at the roots and although its better than nothing at all, you should wash it!
If you straighten it a lot (I do) and don't want to fry it entirely but can't make it last for more than two days, take a comb, wet it and run it through the greasy areas (like the bangs, the hairline) and then just blow it/straighten it :)
Dry shampoo! It's great. Or if you don't have that on hand, baby powder or cornstarch will also work - just sprinkle a little in near your roots, let it set for a minute or two, and either brush it through or just kind of fluff it out til it blends in.
Or if all else fails, a headband and a little bit of perfume to make it smell good.
A lot of the other girls suggested this, but dry shampoo or baby powder are good for soaking up the oil. Also, updos hold better with dirtier hair than clean hair, so put your hair up and it'll look stylish and no one will know it's unwashed.
Tying back in a tight ponytail is the best way to cover up any greasy look. You can use slides to decorate if you feel like it. As for smell (I don't think this is generally a problem but given you asked) spraying perfume (I use Body Shop Vanilla) on your hair brush and brushing it through is the simplest way.
shake a little baby powder onto your hands, rub together, then run your hands through your hair roots. it makes it smell better and look/feel less greasy. or you could use dry shampoo which does pretty much the same thing, and lots of dry shampoos have nice perfumes in them.
use some leave in conditioner with a good perfume (like those cheap cons from v05) and wear your hair up