"Please wait in the hallway."
In Kunrei-shiki, the system taught in Japanese elementary schools, the long o is romanized as ô, so under that system it would be rôka.
In Hepburn romaji, which is what I use and which is taught in English classes in Japan as well as many Japanese classes for English speakers, it would correctly be written as rōka, but the Microsoft international keyboard that I use does not have an option to type macrons (I had to copy and paste ō from an internet search). It's not convenient or practical for me to use a macron, and I find it helpful for learners to distinguish the difference between the long おお and おう which are both romanized using ō with Hepburn. If I had an easy option to type a macron I would consider adhering to Hepburn, but I don't, and other people using a computer don't either, so I find in daily communication most people rarely use macrons when typing in romaji.
What you are suggesting is one of the possible ways a long vowel could be romanized under the Nihon-shiki system, but that same system also recommends ô and ō, and to me romanizing おう as oo could only ever cause confusion, as it does not help pronunciation (if I saw "rooka" I would pronounce the "oo" like "oo" in "moon"), and it implies that the long vowel is written as おお in hiragana.
Romaji has many purposes. One is for people to know how to pronounce the word. One is for being able to type Japanese using an alphabet-input keyboard. One is to check the pronunciation of a kanji or kana that you've forgotten. My purpose in including the romaji at the end is for users who have difficulty reading to be able to read the sentence quickly and easily. I've tried just posting kanji and hiragana, but some users find it difficult to read a string of hiragana, so for now I'm continuing to include Hepburn romaji with just a slight variation for the long o.
Think of に as more concerned with movement such as a physical destination or even an intangible goal (sometimes also an origin but I haven't seen that on duo yet)
The particle で gives context to your sentence. You're not moving to the library. You are simply reading and it happens to be in the library.