i would always play 5 wastes in oath. keeping them off mana for a turn can be the difference between a win or loss.
Yes, before and after you drop Oath, you want to slow their win condition as long as possible. So while they struggle to set up a win condition, you swing away with your Angels.
If you are considering running four colors, just run more fetches, or, even try Rainbow lands (City of Brass) so you avoid ever being mana-screwed on one color. Although, regardless of the fact that your mana base is not looking for colorless mana, you still want to use Wastes as a bit of a Time Walk in Oath.
What kind of build are you playing?
I think a 3 strip if your playing chalice is pletny, it ensure a 1 mana source per a turn. With the strip the opponent can keep a fetch land around until it's needed. In the mirror, strips are definitely not a bad idea, but there are things that are just as effective and multi useful for dealing with the opponent’s orchard, such as engineered explosives and echoing truth. I’ve even used trick bind a couple of times.
I currently run 1 strip in angel oath and 2 Mishra’s Factory main, because in the meta right now, Chalice and Null Rod main deck is incredibly effective.
i'm a little confused at what your mana base is looking like I’ve been kind of toying with a four color build out of amusement and realized that 4x Forbidden Orchard, Mox+ Lotus+ Crop Rotation (+petal albeit uncommon) is 7 or 8 sources of any color you want with out the aid of duels and fetch.
With the type of build it seems your building, I’d run: the obvious 4 Forbidden Orchard, 1 Crop Rotation, 4 fetch, 3 Tropical Island, 3 Underground Sea, and a city. Then strips in the remaining deck space. This brings you up to 8 or 9 red/white sources.
Every once and a while you’ll come across some one saying you should ad duels. assuming you run one duel containing one of each color that’s an additional 5 ways to achieve the mana source you want, in theory. Mechanically it doesn’t work out so well, due to the needing to achieve more than 1 color more than 1 time.