You mentioned two, so Porygon-2....wait.
I have to start with O...O2 is oxygen.
What pokemon relates to oxygen? Plants make oxygen so...
Grass Type!
Grass Type Pokemon that starts with O...
....
.....
Is there a grass type pokemon that starts with O?
Well, in that case, water is made out of H20. It has a 2 and a 0 in it. :P
So...water type that starts with O...
...
....
Are you kidding me? Not a single water type starts with O?
In that case, I've looked in Grass and Water. Both starter types.
The final starter type is fire.
Fire + O...
...
Wait. Just realized there are Water + O's.
Octillery, Omastar, and Omanyte.
In that case...which one?
Omastar and Omanyte are related, so that singles Octillery out.
Hence, Octillery.
Next letter is Y.