If I recall correctly, RoundUp is a Pesticide. Pesticides include
Herbicides
Insecticides
Miticides
Fungicides
Hence Oregon's State Commercial Pesticide Applicators License,
or State Pesticide Consultants License.
FOR THE ORIGINAL POST -
What do you want to do with the land?
Unless I was going to plant a crop, it might be a benefit to save grass type plants for reasons like erosion control.
RoundUp kill almost everything - but for Poison Oak, you will almost have to spray that in mid or late September. And you will need the dust off the leaves. Dust is very bad for effective RoundUp use.
Crossbow may do quite well on Poison Oak.
Also, products like Trimec can wreak havoc on weeds and Poison Oak but at the same time preserve the grass - Trimec is a turf type product. It will kill weeds, big trees, small trees. Mainly via foliage.
One chemical in it, Dicamba, can remain on or in soil, but not to any great extent in one application is all you do.
You may need to use 2 chemicals separately. Some are compatible also, should you choose simultaneous use in areas.
On the Country Clubs, out side the rough, and at University campus wild areas bordering parking lots, we usually relied on at least 2 pesticides.
If you time it - no matter what spray - for after a cleansing rain, that is much in your favor. Be aware of neighbors.
If there are vineyards next door, avoid breezes, ESPECIALLY with 2,4 - D products.
Deodar Cedar, and other Cedrus, take a hit from that too.
You might consider spraying at 75 degrees or cooler, like in the evening, or very early morning.