O disk = low brake caliper clearance
A disk = standard caliper clearance (clears most factory 32,33,34 brakes)
R disk = high caliper clearance (eg: brembos etc...)
Wheels with the same width and offset come in different disk types hence your problem. A lot of ppl have the misconception that just because they had wheels of XX offset before it should fit no problems...
Hope that helps...Like you, I learnt the hard way too..tried slip on spacers, bolt on spacers...considered grinding calipers...in the end if was better to sell them and buy correct wheels to fit. Now everytime I buy 2nd hand wheels I make sure I find out the disk type as this was particularly important especially 34 front brakes.