DNA replication... which nucleotides are most likely to occur at oriR?

You are on the right track. The TATA box is rich in adenine and thymine which form two hydrogen bonds when paired. Guanine and cytosine have three hydrogen bonds when paired, so G/C rich sequences are harder to unzip. An A/T rich sequence is likely. Also, remember that DNA polymerase starts from an RNA primer. I hope I've given you food for thought.