Fast growth requires repotting?
 

Every orchid in my house responds to Spring with crazy fast growth of roots, babies, or spikes.
I do worry about the pot size for the two oncidium shown here. You can see how large the mature pBulb can get. Given the position of the new shoots when should I repot? Thank you.

If the medium is still good you can let that one growth cycle stay in the pot, then repot on the next growth cycle. Otherwise you could repot now.

Leave enough room in the new pot for 2 years of new growth, about 4-6 growth cycles. Wait until you see new root nubs forming from the newest growths.

Do you know how long it takes for a baby to get to the size of nature pbulb? That will determine how urgent repotting is, right?

Also the one on the left has six mature pbulbs in a 5.5" pot. Should I split it into two plants?

A new growth will probably mature within the year.

As far as dividing is concerned... I'd suggest going to a larger pot, and keep it together (at least until the next repotting) Better to have 1 strong plant than 2 weak ones... When going to larger pots, I like bulb pans - shallow pots, so that you can increase the diameter without adding a lot of volume. Also, gives a lower center of gravity so less likely to tip over. (You can find these online) That might be a good candidate for an 8 inch bulb pan.

If you can't find a bulb pan, another approach with a larger pot is to invert a small pot in the middle below the middle of the plant. This takes up volume and creates an air pocket, so that you don't end up with a wet dead airless space in the middle.

Personally, I’d repot it now, as roots will likely soon emerge, and will be very easily damaged.

Dilemma.
I know the new shoots are growing fast against the edge but the spike is shedding happy sap. I would hate to lose the spike again like with Coelogyne.

You can wait until the blooming is done. A growth tight against the pot is no problem... if it doesn't have room, it will likely just expand outside the pot. You can always cut the pot if it is too tight to get out. Orchids tend to like tight shoes.