The various columbariums (with many plaques close together) and the main mausoleum have already been done MULTIPLE TIMES.
St. Thomas Cemetery, which was taken under eminent domain in 1963, had any remaining, unclaimed remains moved to OMOS. There are now several hundred unmarked, unknown graves in Our Mother's Old St. Thomas section as the burial records were lost at some point. If the family paid additional fees, identified remains moved here may have a marker and/or be in a different section. Names shown on the Old St. Thomas plot map but without a marker are those that had a marker in the old cemetery, but no fee was paid for a new one. Everything else is unmarked and not identified on the plot map. If an obituary or death certificate says someone is buried in St Thomas, they may well be here but unidentified. OMOS will tell you they know all the St Thomas graves: it is true they know WHERE they are, but they don't know WHO they are. (The more experienced employees will tell you this. The newer ones may tell you "they're not here" if they don't have a record.) They could also have been moved elsewhere by family: quite a few ended up in Mt View cemetery (where there are records of removals from St. Thomas and Hillside, among others).