Does it mean it was different from what it was in the start, or that from the start is was different from an implied before?
Ó implies a separation in time or distance, so ón tús really only carries the "since the start" meaning of "from the start", and difriúil ó shouldn't be read as "different from" in the sense of "X is different from Y".
Here are some examples of phrases from the NEID that use "different from" in English - you can see that there are a number of different structures that are used o express this idea, and they don't use ó:
"they're very different from each other" - tá siad an-difriúil le chéile
"understanding a language is different from speaking a language" - is mór idir teanga a thuiscint agus teanga a labhairt
"this plate is different from its fellow" - ní hionann an pláta seo agus a leathcheann
"it's what makes them different from the other banks" - is é sin an difríocht idir iad agus na bainc eile
"writing books is little different from writing essays" - is beag idir leabhair a scríobh agus aistí a scríobh
"it's only marginally different from the old one" - níl ach difríocht fhánach idir é agus an ceann deiridh
"it's different from the normal run of films"- ní hionann é agus na gnáthscannáin
"his first books are tellingly different from this one" - tá sé suimiúil go bhfuil difear mór idir na chéad leabhair a scríobh sé agus an leabhar seo