Understanding Variable Scoping in SPARQL Graph Patterns

SPARQL question: It appears that variables can't cross graph pattern boundaries. This doesn't work (Error 400: Bad Request):

WHERE {
	?s :p1 ?o1 ;
	   :p2 ?o2 .
	?o1 :p3 ?o3 .
	OPTIONAL {
		?o2 :p4 "some literal"
	}
}

i.e., ?o2 seemingly can't cross the OPTIONAL boundary. This does work:

WHERE {
	?s :p1 ?o1 .
	?o1 :p3 ?o3 .
	OPTIONAL {
		?s :p1 ?o2 . 
		?o2 :p4 "some literal" .
	}
}

I had to both remove any reference to ?o2 in the main body of the WHERE clause and repeat the s :p1 part in the OPTIONAL clause. Is this because the basic graph pattern and optional graph pattern are completely separate (as far as variable scoping)?