Welcome Todd!
Birch is a sensitizer (makes you allergic over time), skin irritant (it has methyl salicylate or however it's spelled; it's the same thing as in some topical analgesics a la aspercreme), and it's a no no for pregnant women because it's a stimulant. I don't use it. Always ask your suppliers for appropriate usage rates for various EOs or FOs. If you don't have access to the supplier info, then you can try looking up the rates recommended by the International Fragrance Association ().
Oak moss has similar notes to birch. Oak moss is used for its earthy, smoky and leathery notes. IFRA has usage rates for it in various types of products. I have a bottle of it and use it every now and then, though mostly in blends.
If you're going to be using a lot of EOs, Tisserand has a great book on EO safety that's great to have as a reference (). It's pricey, though. During my essential oil training, many of the doctors taking the classes were hauling this volume around with them. Several of the docs were cosmetic specialist types who wanted to understand EO formulating to create their own lines of cosmetics for their medi spa practices, which was interesting to me.
I lookup each EO before I use it now. Irena has taught me that, as well as not to make medical claims and hell, about half of what I know about soaping. Tammy here is an expert on EOs, also. We learn by doing. Grats on your batches of soap!