Is it possible? How does it work?
Many autistic students feel a mixture of enjoying the social environment at school, and also being overwhelmed by it. It seems to some that the ideal solution might be to have a student take certain classes at school and learn other subjects at home.
However, Oregon laws and district practices have not caught up with demand. Currently there is a patchwork of opportunity that must be negotiated by parents on an individual basis.
Independent Homeschoolers
If you are independently homeschooling (not with a charter), you can approach the staff at your local school to see how willing they are to make arrangements. Oregon law allows them leeway in implementation. Some districts have a written policy about this, others leave it up to the principals. They may require an IEP evaluation and documentation, or they may not. Usually for non-core classes such as band, choir, PE and art, there will be less need for an IEP, but it depends on the child's needs.
Extra-Curricular Activities
Oregon law allows homeschoolers to participate in Inter-Scholastic activities, such as sports, band, drama, etc, with certain restrictions.
For independent homeschoolers (non-charter), it is more straightforward, you can sign up, pay whatever the normal fee is, and meet the academic requirements.
For charter school students, there are newer laws defining a way for the charter to pay the local school a negotiated fee to participate in their sports and activities. Since the laws are new, only some of the charters have actually worked this out yet with their districts. See this extensive explanation on the Oregon School Board Association web site.
Special Education Services
If you would like the district to provide special education services, aside from just attending a class, then this will require the IEP process. For some families, the school-provided services are sufficient, others find that private therapies are more helpful and comprehensive. It will be up to you to negotiate with the individual staff at your local school or district to work out the details. If you are with a Charter School, then the therapy needs to be provided by the Charter (for financial reasons, you can't be enrolled in two schools at once).