While Oceans may not be placed, there is no hard limit on Greenery Tiles or City Tiles. In fact, the placement of Greenery and City Tiles is one of the main ways to spend end-game cash for end-game points or milestones.
That's right. Greenery tiles can continue to be placed after the oxygen track has reached its limit, but the player does not receive a TR point because the oxygen level can't be improved.
Greenery placements after the oxygen limit is reached still matter in the end game scoring because (a) they are worth 1 point each, and (b) each greenery placed adjacent to a city increases the value of the city by 1 point.