The answer is maybe you can place market and maybe you cannot. It really depends on your individual situation and your broker.
For most people with an online broker such as Etrade or ATrade, you can most likely place a market order to sell OTC, but not to buy
With a full service broker that charges you an arm and a leg for each transaction, you can likely place any kind of market order on OTCs
For the more active online traders that use Etrade Pro, Atrade Apex, MyTrack or similar services that will allow you to route your own orders to the ECN or MM of your choice rather than just relying on the broker's default routing, the answer is yes, you can use any type of market order you want
At least this is my experience, but then again, it really depends on your broker and I haven't used MyTrack or Atrade for quite awhile.
KTF
"You can't be a real country unless you have a beer and an airline - it helps if you have some kind of a football team, or some nuclear weapons, but at the very least you need a beer". F Zappa