O'Smach entry is still straightforward. You might be able to avoid customs altogether as the little hut is often unmanned. If you want to be sure that it won't be manned, enter either very early in the morning or late at night (although it's often unmanned around lunch time too). A good strategy might be to enter around 9.30pm (the border closes at 10pm) and then ride/drive straight on through. You could also enter officially and wait until after 10pm to ride past the hut or stay in the casino overnight and leave around 5am. Obviously all immigration procedures should be completed on the day you enter.
At Poipet as long as you keep your motorcycle/car out of sight you could probably still make it through without being noticed. Park your ride at one of the casinos, finish up all your immigration paperwork then go back to fetch your vehicle - best is entry after dark especially between 6pm and 8pm during the dry season and 7pm and 8.30pm during the wet season around rush hour. Follow the other vehicles closely, once you're past the gate and traveling around the roundabout, you're in! I see Thai cars driving within Poipet everytime I go there, not sure if they are known to customs and have special connections or if it's the same deal as on the Thai side where locals are allowed to drive/ride Only within the border town. With foreigners they assume you want to drive/ride around the country hence that's why they are more likely to restrict you. That is if someone at the barrier sees you. During rush hour, the gate remains lifted for all the traffic to pass through.
In any case, only try this Poipet method I have described if you have spare time and a backup plan. You could, if you are denied just keep your ride parked at one of the casinos then continue into Cambodia by taxi. If you don't like that, head back to the Thai side and enter at O'Smach.
No problem driving/riding anywhere in the country including Phnom Penh from O'Smach. You may be hit up for a fine if driving/riding along Monivong or Norodom boulevards during the day (there are no night time checkpoints). Try to avoid these roads while you're there and you'll be fine.