OK let's try this
When you "activate" a unit you have "2" (two) things to do with that unit. A bonus move (1) and a standard action (2). A standard action can be 1 (one) of 17 (seventeen) actions. These are listed on the back of the Advance Reference card (column one). You only get one standard action so you must choose one from the list. Move, Fire and Fire-on-the-move are all choices for the standard action.
Think of this as giving "orders" to your unit. As a player you should decide your OM123 option before activating the unit. Then the unit completes the "orders" as given before it begins.
"It is my activation and I decide for my Standard Move to Fire. I am not moving. It is fire from stationary ."
There is no option to make "Standard Move to Fire". You gave the unit orders to Fire-Halt (stationary). You are not moving, this would be an OM1 modifier.
"After seeing the result of the shot, I might decide to use the Bonus Move."
You have already given orders (Fire-Halt) thus the unit cannot move. You have sacrificed the Bonus Move to gain the better chance to hit (OM1).
"Was the shot OM1 ? At that point I had no inkling I was going to move after the shot."
Yes the shot was an OM1 modifier to the "To-Hit" roll. You're decision to move after the shot should have been at the beginning of the activation.
"Or should I declare a the very start: ok I am going to fire but I might move after too. OM 2 then"
Yes, if you are going to shoot and move the unit then use the OM2 (or OM3) modifier. As a courtesy I tell my opponent which of their units are being targeted and which OM value I'm going to use to calculate the "To-Hit" number.
(another wrinkle to your last comment. You decide to shoot and move. OK that means you use an OM2 (or OM3) modifier. BUT, after the shot you decide the unit is still in a good position and want to stay there. This is allowable. You took the move penalty to give the unit the option to move but decided it wasn't needed. This is ok.)
Also remember that a vehicle that pivots (turns in place) more than 45% requires a move action. (MPC page 2.6)
Any clearer now?