Without any knowledge of how vaccines work, it's pretty trivial to determine that there are significant differences between AZ and J&J vaccines; J&J only needs one dose while AZ needs two. That alone implies that there are differences in technology or targeted protein, even if the specifics aren't clear to a non-expert.
There are essentially no differences, except J&J's platform had been successful in an European Phase III trial for an Ebola vaccine, but not the one widely used in the last big outbreak, and Oxford's had an 8 year history of Phase I failures. Both replication deficient adenovirus vectors, with the spike protein spliced in, although maybe significantly different versions of the protein.
The biggest difference is their goals, Oxford was intended to be a regular vaccine, J&J intended to get the very best protection from a single vaccination, with an initial goal of protecting a billion people in 2021, now increased to 3 billion. Besides J&J's press releases and slow but sure development methodology see the difference in their Phase III trial primary endpoints, here's J&J's https://www.jnj.com/coronavirus/ensemble-1-study-protocol:
To demonstrate the efficacy of Ad26.COV2.S in the prevention of molecularly confirmed, moderate to severe/critical coronavirus disease-2019 (COVID-19), as compared to placebo, in SARS-CoV-2 seronegative adults
As in, not stopping people from getting the disease, that's a secondary objective.
AZ/Oxford will be hard to track down because they did several different Phase III trails in even more countries. I'd start with ClinicalTrials.gov, look for Primary Outcome Measures and this is a good search to start with: https://clinicaltrials.gov/ct2/results?term=ChAdOx1&cond=Cov... For the US trial:
The efficacy of 2 IM doses of AZD1222 compared to saline placebo for the prevention of COVID-19
J&J is also doing a 2 dose 8 weeks apart US based Phase III trail to see what that can accomplish.
