Merge the next line if the current line contains a pattern at the end - awk

Question

I have a very big text file. I want to merge the next line into the current line if the current line has a word OR in the end.
Eg. Like in the lines below
somerandomstring OR
someotherrandomstring
The above 2 lines should become
somerandomstring OR someotherrandomstring
Only those lines should change. Rest of the lines must be kept as they are. Thanks in advance.
Allow me to extend the question a bit further.
I want to also see if the next line starts with OR and the OR is not in the end of the current line, then how to achieve the above case and this case together?

Answer 1

With GNU sed:
sed '/ OR$/{N;s/\n/ /}' file
Search white space followed byOR at end of line ($) and if found then read next line (N) to pattern space and replace newline in pattern space (with s///) by one white space.
If you want to edit your file "in place" use sed's option -i.

Answer 2

You can do this in awk by assigning the "OR" row to a variable and printing whatever is stored in the variable when there is no "OR" found.
awk '$NF=="OR"{buffer=buffer$0" "} $NF!="OR"{print buffer$0;buffer=""}' test.txt
This also works on multiple rows that may have "OR" by concatenating the row to the buffer variable until it finds a non-OR row, prints it and clears the buffer variable.
Another option in awk is to use printf on the OR rows and print on the non-or rows (which is kind of similar to the GNU sed example by #cyrus, but in awk)
awk '$NF=="OR"{printf "%s ", $0} $NF!="OR"{print $0}' test.txt
And this is the same beast, but using a ternary operator within printf:
awk '{printf $NF=="OR"?"%s ":"%s\n", $0}' test.txt

Answer 3

another awk
$2$ awk -v RS='^$' -v k='OR' '{gsub(k ORS,k FS); gsub(ORS k,FS k); printf "%s",$0}' file
x OR y
x OR y
for test input file
$ cat file
x OR
y
x
OR y