Difference between ore and romen

Hi. I was wondering what the difference is between the two tengwar letters for R. When should I use "ore" and when should I use "romen"?

That depends on the mode (Classical, Beleriand, General use for English depending on the accent, etc.) you want to write in, they have different conventions.

Quenya mode? General use?

Either one works

In the Classical mode for Quenya, you use rómen if the r is followed by a vowel, a y or another r, otherwise with óre. The combination rd is written with arda:

rómen , cirya , urra , formen , alcar , arda (rómen, cirya, urra, formen, alcar, arda)

For the general use, it depends on whether you transcribe Quenya, Sindarin, rhothic dialects of English or non-rhothic dialects of English. Most English in Tengwar you will see on the internet use basically the same rule as in Quenya, rómen before vowels, óre otherwise:

red , car , art (red , car , art)

Also, this site has really great articles on the different Tengwar models, you might want to give it a look: at.mansbjorkman.net