hi manthos, OS is one of the few airlines who allow upgrades on board, either against cash or against M&M miles. i never really did the math but i think it is cheaper for once, and second if C is fully booked and they have a last minute no show they will sell this last seat to you onboard, thats how most of my on board upgrades worked...

yes you actually do get the C miles if you bought up, all you have to do is fax them the cash receipt you get on board, this also worked every time i did it...