Is there a rule for repeated letters when creating acronyms/initialisms/abbreviations?
Turn my initialism in something like 'OS3P' is an option, but it seems to be naive and informal.

There's no rule against such kind of initialisms, and I wouldn't consider it naive either. Actually, there are standards designated in similar ways, e.g. the I2C and I2S data busses, also spelled I2C and I2S.