You'll receive an Oceanic Clearance comprising the cleared route, the level and speed to be maintained from Oceanic entry at, for example, LASNO/BEGAS, OMOKO/NAVIX. Carriage of HF is mandatory and check your RVSM approval and MNPS certification re. filed cruising level.
In the case of T9 (both directions) and going south on T16 you should request Oceanic clearance from Shanwick 30-90 minutes prior to boundary ETA. (I suspect Santa Maria will be similar in respect of NAVIX).
Clearance request should comprise estimate for boundary position, requested level and mach no. and, in addition, the maximum level you are able to accept at the boundary. (Should your requested FL be unavailable, it's always good for ATC to know what you might accept as a higher alternative).
Oceanic is Class A airspace and the procedural environment dictates that cleared routes, levels and speeds must be strictly adhered to. Any alteration to these in the enroute phase must be done so via request to ATC, even for something as simple as changing your speed by +/- M0.01.
I'd suggest some more background reading from official sources before you head that way. Good luck.
